That's brilliant! I wholeheartedly agree. Given that developers are going to develop Android no matter what, reduce their obstacle to building WP7 apps will result in lots of goodwill from many Android developers and the opportunity to ride the developer curve. This is truly the "Developer! Developer! Developer!" strategy (though not one Steve B. has in mind).
Well, I guess we'll see how this works out for RIM. They've already announced their "not BlackBerry" tablet OS will allow the sources for at least vanilla Android apps to be easily built as RIM-tablet apps as well (though the packaging will be different; IIRC, you won't be able to just give them a .APK).
This almost exists, it's FOSS and is called PhoneGap. Windows phone support is reportedly "coming soon".