I am not a germaphobe and we've already had H1N1 in our house (or, at least we're pretty sure that's what it was.)
It ripped through our school in October, with over 1/3 of the kids being out on some days. (There was talk of closing the school, but they never did.) We caught it on the "early side" and my kids were over it by Halloween. However, a lot of other kids were sick.
I was *shocked* by how many parents sent their kids out for trick-or-treat with the flu. I even asked some of them about it (since I know their kids had been out of school) and heard answers like, "Well, Johnny was *so* excited about trick-or-treat, we didn't want him to miss it." What?!?!
So, if parents were doing that (and it wasn't just one or two), I have *no doubt* that parents at WDW would pump little Johnny full of ibuprofin and head to the magic kingdom. "Because Johnny was so excited about it..." or "because we've spent so much money..." or whatever.
I am sure that happens all year long, but with the sheer numbers of people who are getting sick this year (Last report I saw estimated 22 million? I think that's low.) I think your chances of having a sick kid coughing on you in line -- or from the next table in the restaurant, or on the monorail -- is higher than normal. WDW is not really a place where you can stay away from others, unless you don't plan to leave your room.
It would stink to miss out on those free days, but I don't think I'd chance it. I know you said rescheduling is not possible, but could you *try?* Maybe you could move the trip *up* by a few months, before baby arrives.
Yup, my neighbor's did this. We live on a small (9 houses) street (court - with a dead end) and so we all know each other. I don't let my kids play with the neighborhood kids as they are constantly sick. We are talking major stuff - RSV, influenza, etc. They know that my son is asthmatic, and spent many times in hospital last year, as well as surgeries. So, what do they do? Bring them to my door for Halloween! Mom doesn't tell me until my kids have already had contact. I was floored, but not surprised. Then my best friend, who lives next door, tells me what the deal is, and her kids (my godson) also have fevers. I was angry and immediately left the driveway and brought the kids in. No trick or treating for them!
I explained to them (the baby is only 2, and so we weren't really going out as his asthma was acting up), that we can't trust people to make the right decisions. My very moral 6 year old started crying, but she got over it as she didn't want to get sick. They had already been very sick in September, and missed weeks of school - even though DH and I are professionals who work full time, we are beyond vigilant and keep our kids home extra days, not only for themselves, but others as well.
Most people I come into contact with are NOT like this. DH and I know that only we can best protect our children as many parents are oblivious, don't care or too busy to pay attention to their children's illnesses. Some legitimately have work or childcare issues, but when you have children, you must have an illness plan in place. I'm a teacher, so you really don't want to get me started on this topic!
I have held my very sick baby (22 months old - much older than a newborn) in my arms while in Arnold Palmer's Children's Hospital when we should have been at Disney. She contacted/had: Strep (very rare in babies), Rotavirus, Gastoenteritis, Severe dehydration and acidosis. The yucky Centra Care Clinic near Downtown Disney, misdiagnosed her, and we were in emergency several days later. Good thing we quarantined ourselves (against that horrible doctor's wishes who said, "Sure, you can take her to WDW. Go and enjoy ourselves!) The emergency room dept docs were very upset with him for the misdiagnosis as well - he swabbed for strep, but that was it! I am not one who believes that doctor's are perfect; far from it. They can and do make many mistakes, and some of them are irresponsible. He was very irresponsible to suggest we go back into the parks as our baby had Strep - we immediately quarantined ourselves in order to protect her, and everyone else's children. Antibiotics for Strep start to work within 24 hours and kids can, if not other symptoms present, return to school. Our baby had a host of other symptoms present, and hadn't taken any meds yet, good grief! Good thing we listened to ourselves, and not him...So, when I hear of a doc saying it's ok to take a newborn on a plane or to Disney World, I don't have much faith in that doc at all. They should know how sensitive and delicate their immune systems are at that age, and should not be encouraging or recommending that parents take their babes into a sea of germs. Maybe it's job protection...?
We were very far away from home as we live in Canada - DH and I were nervous and scared, but we were so well taken care of. She was made sick by irresponsible parents who no doubt didn't want to lose their money and brought very sick kids to Disney. She had just passed a very extensive physical days before, so blood work, etc. was done (it was her 1st real physical) and Chief of Pediatrics in consultation with our doc, absolutely agreed based on incubation period that she caught the illnesses at Disney. No questions about it! They said it's so full of germs - they actually do testing there, so he told me about the kinds of diseases and illnesses they find there, and so if I told you, no one would visit again!
My point is that during certain times of the year, this being one of them, they advise that infants and small children do not visit. This year's cold/flu season is new and scary, so I can only imagine what those pediatric specialists would say (incidentally, we had 2 interns and the Chief of Pediatrics taking care of us for 2 days).
I do wish the OP and everyone else who is bringing babies to Disney World good health. As a parent who has dealt with a medical emergency while on vacation (we went in March, and it's peak strep and rotavirus season in Orlando - had we known this, we would not have gone) at WDW, it is scary, and absolutely not worth it.
We now travel exclusively in the summer and we love it so much better, as our health and the health of most guests around us are so much better than during cold/flu season.
Tiger
The baby is 6 weeks old! I know you can't keep them in a bubble but between the regular flu and the swine flu hitting so hard why on earth would you bring your child to the busiest place on earth with people from all over the world? I could go to a mall and not even encounter a quarter of the people you will encounter at WDW so it's not like I am saying not go out but why wouldn't you minimize the risk? There are umpteen threads about whole families getting H1N1 at WDW. Some of these families even had the first vaccine shot of H1N1 and the kids still got sick. Is it really worth it? Yes, you get immunities from breastfeeding (I BF all my kids) but that does not make them invincible. 
