nope not possible because windows phone 7 is closed source. it would be nice though but sorry it wont happen
you can have android (google "nitdroid") but just without texts,calls and camera and u can have different versions of linux. google them for tutorials and stuff
also windows 95 and older versions have worked but no tutorials so far i think.
and someone managed mac os x but at like 2fps so thats kinda useless...
and also meego is (hopefully) coming soon for n900 after a long wait so we'll see what happens with that