Apple will do the same thing it always does with stuff like this: introduce a system that accomplishes 50% - 80% of what people need and then listen to what people scream the loudest about for where to go next.

First round: Push notifications. This (mostly) satisfies the loudest screamers... "I HAVE TO RUN MY IM CLIENT TO GET IMs!!!"

It's all speculation from here on out, but...

Second round: Background Music. Apple will provide some API so applications like Pandora can play their music in the background. This will necessarily entail some amount of background processing to be allowed, but I'm sure it will be heavily curtailed.

Third Round: Limited "full" backround support. Who know what this will look like, but it will probably be something like the user can select N apps to run in the background

By the end of the third round, hopefully the hardware and battery technology will have advanced enough that Apple will allow any app to run in the background