Hi! I did just complete the DL 1/2 at 34 weeks pregnant (about 7.5 mo). It wasn't as difficult as it could've been, because the weather really cooperated. I wouldn't exactly say that I "ran" it, though...LOL. I wogged the first 10.5 miles, averaging between 14-15:30 min/mile. This included a potty stop around mile 8, too. When I hit 10.5, it was really starting to heat up...looking at the garmin, I realized that I could walk the rest of the way at around an 18 min/mile and still finish under 3:30, so I slowed it down. I think I walked miles 12 and 13 at about 17:30! Jogged the last .1 and crossed the finish line with my arms up and a smile on my face. Chip time 3:26.
To put it into perspective, last year's DL 1/2 (my very first race), I finished in 2:29. My goal this year, however, was just to finish and get my C2C medal! I was already registered before I got pregnant, so I had to give it a try.
Disney does not require a dr. note or anything. In fact, they didn't say anything to me at all. I was also worried about that, but it was ok.
Here's my advice:
1. Tell your doctor from the very beginning what you are planning to do. If you are already in training, and have been running for awhile, he/she should tell you to continue what you are doing, just tone down the intensity and watch your heart rate. Keep reminding your doctor about the upcoming race. Since you will only be 6 months, it should be a lot easier than 7.5. I'm sure that if I had done DL at 6 months, I would have been a lot faster...3:15 or maybe even 3:00.
2. Build slowly. Since you haven't done a distance race before, I'm a little concerned. I had 2 halfs and a full under my belt before this year's DL 1/2. I knew that my body knew the distance, therefore wasn't worried too much about training for distance. I wogged 10 miles 3 weeks before the race. That was my longest distance this time. If you are diligent with your training (distance, not speed), you will probably be ok.
3. Hydrate, hydrate, hydrate. Take water and powerade at every water stop. You will probably need a potty stop or two. Fuel regularly, every 45 min. Be prepared to quit if at any point you feel faint, dizzy, contracting, etc. I had to keep telling my doctor that Disney has a lot of medical tents and I would stop if I had to.
Anyway, I just realized that this is more info than you probably cared to get from me...

LOL Short answer, no, Disney doesn't require anything special from your dr. As long as you keep running and everything looks good at your appointments, you should do great!
good luck!