I'm really sorry about this! Are you on a 3GS, by any chance?
We're still working on figuring out the specifics, but since the new phone released, we've heard from some customers that are having problems like this. (From what we can see so far, the problem is specific to the new hardware, though it seems to affect both new customers and folks that migrated forward from older hardware.)
If that's the case, I know they're working on v1.5.1, but the best thing to do would be to email the
support ninjas and let them know you're having this problem.
If you're not on the new phone, then we'll probably want to get the sync logs from your phone. That'll tell us exactly what's going on on the phone during the sync. The support ninjas can help you with that, as well.