We went with QR codes for initial development because most phones still don't have NFC built in yet. I definitely agree, though. In the long term, NFC will take over. It's on the dock for future versions.
It's still early to push any cloud wallet solution for the mass market. Until any of these services build up trusted reputations, handling the transactions on the phone is the best way to go for the average user. Setting up personal servers works for nerds like us, but isn't a general enough solution.
You can run background processes on both Blackberry and Android. Apple's taken a hard line so far, except for apps they've written themselves. For example, you can run the iPod app in the background and surf the web.
As for the rumored slate... who knows at this point.