If you’re iPhone isn’t synching quickly, make SURE you are not passing through a hub. For instance, I have two USB ports on the front of my system by way of an internal hub – this produced painfully slow syncs, even though it is a USB 2.0 hub. Plugging in the iPhone to the rear motherboard ports of my system made it normal.
For the record, this problem DID NOT occur with my original first generation iPhone when connected to the front ports of my system (weird). I suppose the 3GS/3G does something differently in terms of USB communication. Your guess is a good as mine.
You should see a typical song move from computer to iPhone in 1 second.