Why won't my iPhone sync to my iMac?

I'm trying to sync my iPhone 4 to my iMac using iTunes but every time I plug it in and click sync a pop-up window says "The iPhone could not be synced because the sync session failed to start." I made sure the cable was plugged in right and everything looked normal, in the settings, summary, etc. But it still won't sync. It's not syncing wirelessly anymore either.
If anyone knows the solution to this, that would be great. Thanks.

IDunno.

sorry, couldn't resist.

But seriously, the Apple Store may have some answers. I would go there if you don't get a solid answer here.

You may need to reinstall iTunes.