The apps come with the software upgrade, available to all current iPhone owners. They aren't exclusive to the 3GS.

MMS is coming, (again, to everyone with an iPhone) but ATT has got some work to do first. ETA is end of summer.

Camera is improved (3 megapix), and supports focusing (touch the object you want to focus on.) This will finally enable bar-code reading.

The new phones have more memory at the top end. The 8gig models are getting price slashed (99 dollars with contract.)