Okay...

1) Techically it can be done using techniques called virtualization or emulation. And only by someone who have access to Windows Phone 7 code so they could put it there.

2) Only a handful of people would do it if they could, and only to prove a point. Once they had proven it could be done, they would put a video on youtube and then abandon it.

3) The handful of people who could have done it if they wanted to, probably no longer feel a need to prove how excellent Maemo is, because they're expecting Meego. Meego is a new/merged operating system that Nokia will be using instead of what we know as Maemo. It is in part based on Maemo.

Edit: I just realized I failed to use simpler words.