It would seem that all is not well with the 2.1 firmware update. We've had reports from several users (although by no means all users with the 2.1 update) that their phone freezes while attempting to sync. Reading the comments on the previous article and the support e-mail, it does appear that the problem can be resolved. We're working to determine the exact correct set of steps to un-freeze the 2.1 iPhone and will post it when it's ready. In the meantime it might be prudent to not update your phone/iPod to 2.1 just yet.
We were a little asleep on this issue unfortunately because although there were a few support tickets over the weekend that mentioned freezing, we had seen similar reports trickle in for the past couple of months (from users with earlier firmware) and so for a while we assumed this was just more of the same random iPhone hangs, and not something related to the use of our service. There were more reports on the problem, and more detail in the blog comments here than in the support e-mail, but we were focusing on the support e-mail.
Check back here for updates on this issue.