So here’s the story on multitasking in Windows Phone 7. According to the director of Windows Phone:
The core operating system is a modern multitasking operating system. If you play music for example, the music will play back as you navigate around the experience…if you’re using email, we have great support for push email and that happens in the background.
For third party applications—we’ll get into a lot more detail on this in MIX—but we have a few ways we’re going to make sure that third parties can bring their value to the user even when the app is not running. Live tiles are an example. Data feeds in the hubs are another example for some apps.
It looks like multitasking will work much like it does on the iPhone, but with a little more flexibility. Certain apps will be able to run in the background (meaning, default apps like mail/music/etc) while third party apps will be able to tap into its own tile on the front page (and thus become a data feed) or perhaps make use of a system-wide notification system (like on the iPhone).