I could be wrong on this, but just judging from the little Android development that I've done, the standard behavior is for programs to go into a sort of sleep state when not in focus. Their resources are cached if you need to bring it back, but if the OS runs out of room, it'll start dumping those caches first. The program isn't actually actively running/updating like it would if it had focus. So any additional power draw over the normal cycles of the phone is negligible if anything.
You can sort of experience this by switching to and from update-y apps like twitter and facebook clients. When you open/refocus them, they generally have whatever data you left them with (if it was running in the bg), then upon being refocused, it kicks off a refresh event that updates your view in the first couple of seconds that it regains focus. This leads me to believe that while running in the background, it is only doing periodic updates for notification type data at intervals instead of actively.