While that's possible -- after all, there's an add-on for Visual Studio 2010 that supports Windows Phone 7 development - I'm thinking something else may be afoot. If Ballmer is going to make a cameo appearance at WWDC, I'm thinking if such an announcement happens, it'll be Silverlight for the iPhone.
(Thanks to developer Mike Sax, president and founder of Sax Software, for his tweet that put this idea in my head.)
If this rumor is true, that would be a slap in the face of Adobe, which has had a war of words with Apple's Steve Jobs for not allowing Flash on the iPhone or iPad. There are many media streaming sites that leverage Silverlight, including the popular Netflix website. While Netflix recently announced streaming Netflix on the iPad, Netflix for the iPhone still hasn't arrived. I've also heard through the grapevine that the Netflix app for the iPad has poor video quality (choppy video), and no fast forward or rewind that you get when you use Silverlight. So perhaps having Silverlight on the iPhone would allow for streaming Netflix's video content in its native Silverlight format and allow for higher quality video along with fast forward and rewind.
So will it be a Visual Studio compiler to Apple's native Xcode or will it be Silverlight for the iPhone/iPad. I'm skeptical it'll be Visual Studio compiling. For one, Visual Studio compiling for the iPhone would be a violation of the controversial 3.3.1 developer agreement banning third party compilers. Why would Apple want to enable Visual Studio developers to compile apps for the iPhone?
Although allowing the millions of Visual Studio programmers out there to develop and compile apps for the iPhone would help Apple with the DOJ investigation which claims that Apple is forcing developers to use only it's tools to develop iPhone and iPad apps.
But if you ask me, Steve Ballmer is going to announce Microsoft is acquiring Apple. Ballmer is ticked off that Apple's market cap is bigger than Microsoft's and his notorious ego just can't take it any more.