I had no problem syncing the first time I moved over to leopard, but I did have a problem last night after upgrading my iPhoto and Quicktime. For some reason syncing just stopped working after the updates. The only thing that fixed it was deleting all my stuff and then having iTunes put it all back. It just wouldn't sync at all with the old files there for some strange reason.
If you've got you stuff backed up, try removing it from the phone and then try to sync with a clean phone. Worked for me.
Problem fixed. I found this on another forum. Someone posted that the version of iTunes that installs with iTunes causes the problem and suggested trashing it. I then used software update to automatically find iTunes from the Apple site and install it for me. Next time I launched iTunes my phone sync'd just like the old days. Thought I'd let the rest of you know in case you had the same problem.