Over on XDA-Developers there's a thread where user mightymn was able to develop a program that keeps counting even when the emulator is switched back to the start page or a different program. That would seem to indicate real multi-tasking capabilities in Windows Phone 7. Unfortunately, it turns out he was using the April version of the Windows Phone 7 developer tools/emulator and the latest version no longer shows this capability. (You might already know that.)
Oh well! Maybe in the future there will be a mechanism for enabling certain apps to having the multi-tasking capability. Microsoft has mentioned that they will evaluate multi-tasking for certain apps on a case-by-case basis, so all hope is not lost.
Meanwhile, have you asked yourself "How exactly do you multi-task?" It may be that most of your multi-tasking scenarios will already be covered in Windows Phone 7.