Stevent Kent interview Joel Hochberg, and found this out:

Interestingly, though Rare is now owned by Microsoft, it will still have some dealings with Nintendo. "We still plan to work on games for Game Boy Advance in the future," said Hochberg. Rare has developed Game Boy titles in the past, and Microsoft does not currently manufacture a rival handheld system.

Hurray, Rare hasn't completely left us for Xbox after all. Shame about Kameo and Perfect dark Zero though.