I couldn't get my iPhone to backup to my new Macbook Pro after using Migration Assistant. There are several threads about this on the Apple Support forums but the solution that worked for me was re-naming the backup folder. The folder is in user/library/application support/mobile sync.
It might come in handy some day. I just changed it from backup to backup1.

Common sense and deodorant are a lot alike. People who need it the most don't use it.