And since it looks like Apple is betting the company on the iPhone/iPad (as opposed to Mac OS X) there's no real downside to letting developers on non-OSX platforms build iPhone apps.
You would still need the $99 Developer License to load apps onto the device. And Apple gives Xcode away for free so they're not losing money on allowing competing development tools.
I still don't think this announcement is going to happen, but it's not the most outrageous thing that could happen.
If anything its more likely that Microsoft would reject such an idea than Apple.