Its my opinion that Apple will want to avoid showing any preference to either US carrier, and will most likely release the iPhone 5 for both AT&T and Verizon on the same day, even if that means pushing the release date a little further out to compensate. This also says to me that the iPhone 5 will be an incremental upgrade, similar to how the 3GS was just a faster 3G in the same form factor. The CDMA iPhone required an antenna redesign, and it doesn't sound like Apple wants to engineer a new phone just yet. They are probably waiting for LTE to take off and build a single world phone that works on both networks everywhere.
I'm going to wait, since I'm still under contract for the Droid X until June. But if the iPhone is something you want and you're ready to upgrade, go for it. It'll still be a great phone six months from now, even if a new model is out.