I am unable to load music onto my iPhone. I've already restored the iPhone once, thinking that would fix the problem. It didn't. I am able to load music on an iPod nano just fine, but when I drag music onto the iPhone the selection just bounces back.
The only person I know who has one has not yet tried this yet, so I don't have confirmation from a reliable source. He's still having SIM issues. I would say take it with a grain of salt, but try it and see.
EDIT: There is a whole thread about this at iLounge:
Option 1 - create a playlist and only sync that one (or create a few and sync them all)
Option 2 - place check marks in front of the songs you want on your iPhone, then on the iPhone screen at the bottom check the "sync only checked items" box
Option 3 - check them all and let them all go over and/or uncheck the sync only checked items box and the entire library should sync over