Status of using same WinXp partition for bootcamp and parallels?

Hi,

I think I'm not alone in wanting this scenario:
1 partition on the Mac disk with WinXP installed via bootcamp
same partition usable as image within parallels under Mac OS

There are a lot of posts about problems with this, so I'm wondering if anyone actually has it working? If so, what is the magic to make it work?

Btw, by working I mean not having to re-register Windows whenever switching between dual booting and parallels!

Thanks much in advance!