This solution from Get Satisfaction worked for me. I had a total refusal to sync until some direct to phone downloaded podcasts were deleted.
In the end my issue was related to podcasts that I downloaded directly to the iPhone. This created a glitch of some sort. Once we removed the podcasts then the iPhone would be recognised in iTunes and would sync.
My iPhone then sync'd successfully, wirelessly even, for about 3 days before the inevitable iTunes Sync Error.