Quote from Engadget, to me this seems like true multi-tasking, but I don't have the facts.

Microsoft indicated it didn't previously allow for third-party multitasking due to battery life concerns, but those concerns have been mitigated -- somehow. We're not sure of the API-level details that's letting all this magic happen, but we'll look for those later. All we know right know is that it looks great and we can't wait to try it out for ourselves.