I'd be absolutely shocked if this happened but after thinking about it, it would actually make sense.
The amount of extra money Apple gets from developers buying a mac just to develop iphone apps must be minimal. The reality is Apple mac sales come from people who are either design based or consumers who just use software. Allowing windows users to develop for iphone/ipad is a win win for Apple and MS. It will accelerate app development on the platform and really f*ck with google.
For MS it's going to be a weird concession to make. Realistically, MS is saying we have no hope of producing a competing ipad like device in the forseeable future so we might as well partner Apple on this. It will also signal a departure from MS's strategy of "finger in every pie" to "lets just eat well" and have them focusing on the core businesses.
As much as Xbox is popular etc.. what has it done for them? They make a bit of money now but lost billions doing it. I dont think Apple have the appetite for that again. Apple have shown that you can grow a company without losing money in the short/medium term. If MS never made anything other than Office + Windows stuff they would probably have more dough and be more profitable. Thats the reality.
Now if MS used with webkit for ie9 and Apple used bing as default search I think google would be petrified... The message would be "dont fu*k with Steve Jobs, you have no idea how far he will go to destroy you"..