I'm waiting too...
My Verizon contract recently expired, and now I'm waiting for the next iPhone to come out before I switch. Hopefully it will be in June...while I'm currently using a POS razr as my cell phone, at least I have my iPod Touch to hold me over if it ends up being a longer wait...
While I'd like 3G, I'm also holding out in hopes of other potential features like 32 GB of storage, a better camera and/or video recording capabilities, and improved battery life.
GPS would be nice too, but I already have a standalone GPS that I like to have mounted on my windshield (convenient when driving), so full-fledged GPS in the iPhone doesn't interest me much. The current 'fake' GPS is good enough for my current needs (I've used it on my iPT).
If I thought 3G was going to be the only update I'd probably just buy right now. But I'm optimistic there will be other updates along with 3G!