If Microsoft wants Windows Phone to have any chance of becoming a success, it needs Nokia (and others) to choose WP as their only software platform.
Buying the Nokia mobile division is probably to only way they can prevent Nokia from adopting Android in 12-18 months.
If the first Nokia WP phones aren't a big hit immediately from the start, the market share of Nokia in the smartphone market will have dropped to such a low point in 12 months time that shareholders will insist that Nokia management should "do something" .
And the only thing Nokia can do at that point in time is to place bets on more than one horse, so they will probably announce a future Android Nokia device in addition to WP7 somewhere in 2012.
Unless Microsoft buys the mobile phone division....

So yes, I think this is very likely to happen actually.