Recently, I accidentally grabbed my wife's iPhone and plugged it into my MacBook Pro. No big deal, right? Nothing bad happened. But after that: iTunes refused to sync my calendar, contacts, e-mail, or bookmarks.
Watch this: Restore iTunes sync
I found a fix on the Apple discussion boards suggested by StephenDunn2. Be warned, Apple specifically states you should NOT mess with the SyncServices folder. As if it were a swarm of bees, they say you should avoid it.
OK, I get the picture, but at a certain point, if you're frustration surpasses your bee fear, you may want to try this. It worked for me. Here's what I did.
1. Quit iTunes and, if necessary, Mail, iCal, and Address book.
2. In your home directory, find Library, then Application Support, then SyncServices.
3. I moved the SyncServices to my desktop as a safety.
4. Then restart iTunes.
5. Go to the info tab and reset your sync preferences.
6. Then sync.
If this works, you should no longer get the sync warnings, and you can delete the old folder off your desktop.
Remember, this worked for me and plenty of people on the boards, but not every single one. And Apple certainly doesn't like it, so there's no guarantee.