Try restarting iTunes and your iPhone. For iTunes, close the app and restart your computer. For your device, press down on the Home Key and teh Sleep key at the same time and hold them until the silver Apple appears (ignore the red slider bar).
Once both have been restarted, try syncing again.
If that doesn't work, go to the music tab and unselect all music and sync. Then re-select the music you want on your phone, and sync again.
Hopefully one of these solutions will work for you!
I'm running OSX 10.5.8 on my PowerBook, and after updating to IOS 6 on my iPhone 46, the phone wouldn't sync using iTunes. iTunes said it was up to date, but I didn't have 10.6.3, which is what the error message kept saying I needed. After fishing around on the Apple Web site, I found the version of iTunes I needed and that fixed the problem. Go to downloads and search for iTunes 10.6.3, if you're running an older version of OS X on your computer.
If you're running Mountain Lion (Mac OS X 10.8), this is what worked for me:
goto the support link: http://support.apple.com/kb/HT1747?viewlocale=en_US&locale=en_US
Note: If the mac tells you that you can't remove iTunes, you will have to do a "Get Info" on the iTunes application icon (command + "i", or right click on icon and select "Get Info") and change the "Sharing and Permissions:" (click on the lock at bottom right, enter your administrator password, then add adminstrator (use "+" sign on left) with "read & write" privilege for the "admin" name, so that you can remove iTunes prior to reinstalling it as described in the support link. Make sure you follow the steps as described in the support link. (However, it is not necessary to delete everything from the trash, as described in the support link, at least prior to completing the re-install, in case you screw things up and want to go back to what you had).
At least for me, when I reinstalled iTunes all the preference settings were nicely maintained. Good luck.