I am trying to sync my iphone 4. Get error :"itunes cannot sync apps to the iphone because the apps installed on the iphone cannot be determined"

I wanted to sync my iphone 4 (cdma). I have synced it before successfully. I am getting an error "itunes cannot sync apps to the iphone because the apps installed on the iphone cannot be determined" and the apps and items follwoing apps in syncing (photos for example) will not sync. Items prior to apps synced (calendar/contacts). I restarted both my iphone (currently on ios 4.2.8) and my computer, itunes was on 10.6.1, same problem. I installed itunes 10.6.3, same problem. I had been waiting to update ios to 5.1.1 and would like to do that now, but wanted to sync first. Anyone have any ideas? I am stumped! Thank you.