This is the only source I have but even this doesn't sound like it's really going to happen.
"I hate to lend credit to this ongoing rumor, too, but I figure I can't ignore it. At least one industry insider I spoke with brought up the whole Microsoft/Nintendo union; apparently Microsoft -- more than Nintendo -- is interested in some kind of partnership."
It came out of IGN's mailbag, so don't expect it to really happen...
http://cube.ign.com/mail/2003-06-26.html