Report on my experience: I bought 4 tickets through Orbitz for a family traveling with us, and then I saw that ebates on Expedia was higher, so I bought our 5 tickets through them. They both sent me emails with voucher numbers, but they are not the numbers that need to be entered into MDE. I wasted a lot of time on the phone with Expedia (one hour) and then with Disney (20 minutes) the day after I bought them to try to get the MDE linking numbers, until finally Disney told me to call 407-566-4985 Option 5. I waited another two days before trying again, and by that time, I got an email from Expedia with the numbers in it (not the voucher numbers, the 12-digit numbers that started with the first four letters of my first name). I entered those numbers into MDE and they worked. I don't know if I would have got the email with the ticket numbers if I had not called, but they did not reference my call in the email.
Once I'd done all that, I still had to deal with the Orbitz tickets. For them, I started by calling that Disney ticketing number, Option 5. I got right through to someone who knew exactly what to do, and she was able to give me the numbers (again, starting with the first four letters of the first name) and I entered them into MDE myself and they all worked fine.
My ebates from Expedia have come through, but Orbitz is still showing as pending. So from my experience, next time I would do Expedia over Orbitz. They have higher ebates (at least at the moment), they send an email with the numbers (though it took two days), and the ebates came through more quickly. Either way, it seems the Disney ticketing, Option 5 people are the best ones to talk to if you have any trouble with linking to MDE. And next time I know to do it at least a week before I want to do FP+! (Though I did end up getting everything I wanted.)
I've done UT before, and I know that they are great, but our ebates totaled over $120 for the two families, so that was worth some extra work for me.