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!!