Obviously the iPhone already has multiple processes running concurrently (e.g., phone, texting, iPod, email, even web pages seem to continue loading after I've quit Safari). It's a busy little machine. I don't want it to get slow. Letting users decide (kind of like location-based services) would be a nice feature.
Um... has no one experienced the many, many apps that run as backgrounded in a jailbroken iPhone?
For example, would you like your weather icon to display the *current* temp on the home screen? Done and done. There are so many great examples of this (Intelliscreen probably being the best - and one of the first, interestingly enough!) that it's silly to talk of "can the iPhone do it?"! It's *been* doing it for thousands (probably hundreds of thousands) of people.
C'mon Apple, get on board the "we know what an iPhone can do" train!!
TV that was next to useless since it crashed so much, and the iPod touch that had iPhone errors constantly.