...the only Rare-developed title on Virtual Console right now is the original Donkey Kong Country' but the Donkey Kong series of games is a Nintendo-owned franchise starring an established Nintendo character, whereas something like Banjo isn't'so it's nowhere near as easy to call. Especially when you also consider the existence of Xbox Live Arcade. So right now, the best answer we can offer (for either download service) is 'it's possible.'
Not a solid answer, but not an out and out 'no' either. Hopefully Rare, Nintendo and Microsoft can come to some kind of agreement on this situation - there's nothing stopping them from releasing games on both the VC and XBLA. The fact that games like Banjo Pilot (GBA) and Diddy Kong Racing (DS) have made their way to the market bodes well for a working relationship between the three companies. Surely Rare and Microsoft wouldn't be foolish enough to turn down this opportunity to make cash with very little effort involved? We shall see.