Problem solved. Googled the log entry and it appears that if you install or update Xcode (Apple's development environment), a lot of the mobile framework is deleted. The result is the symptoms I originally described. Update Xcode from same same DMG and it all comes back. Since I had updated Xcode maybe only a day or two before I discovered I couldn't sync, I ran the update again. And yes, it all came back. This is an Apple oopsie worthy of Microsoft.
Thanks for the useful info.
This problem is still around - OS X 10.6.8
Reinstalling Xcode 3.2.6 for Mac OS X 10.6 and iOS 4.3 fixed it.
It is better to quit iTunes before being prompted to.
Otherwise Installer hangs with 2 minutes remaining and needs killin'.
The prompt box still remained so I re-logined :-) and did the whole thing over