The beauty of running OS X on it is that it runs smoothly to begin with and if there are any problems, they could just release an update to fix it. The phone would prompt you to update your software and it'll all be done in the background. The only thing "new" or "untested" is the touchscreen. I won't count the proximity sensor because that's nothing new. So, it's a $500 plus bet on the screen for you and it's a bet worth millions of dollars for Apple. I doubt they are going to fuck this multi-touch thing up, because we will start seeing this technology in other Apple products gurantee.
They didn't go with 3G because most of the targeted consumers wouldn't know the difference. But it's almost for sure, from looking at Cingular's efforts to expand their 3G network, and reading the rumors that 3G iPhone will show up. They would have to if they're trying to move into Europe and Asia as well.