A number of users are reporting problems with synchronizing their iPhones after applying the recently release Mac OS X 10.5.2 update. Issues include iTunes not recognizing the iPhone, error messages during sync, or incomplete syncs.
Failing that, try deleting the following files:
then reinstall iTunes 7.6.
There are several other workarounds for sync issues, including connecting the iPhone directly to a computer's USB port rather than through a USB hub; switching USB ports if you're having problems with one and more. See this article for more.