As we approach the June WWDC event and the expectation that Apple will roll out iPhone OS 3.0 along with some possible new hardware, the rumors of what else may be in the OS update heat up. The latest speculation is that Apple will allow one or two 3rd party apps to run in the background. Currently, Apple lets you run the iPod music functions in the background, but no 3rd party apps have this capability. The servers side notification support coming in iPhone 3.0 may address the needs for some apps, such as IM apps, but there are still some others that would benefit from running in the background on the iPhone.
I think the most valuable type of application to run in the background for me is a streaming radio app like Pandora or imeem Mobile. I hate that I cannot enjoy streaming content while surfing in Safari, writing emails, or checking out Twitter or Facebook. Looking through the screens of apps I have loaded on my iPhone, if I had a second choice (assuming my IM client was working with the upcoming Apple server technology) I think it would be a Twitter or Facebook app to allow me to stay current with status and updates.
If you could have one or two apps running in the background on your iPhone, what would you choose?