Earlier today, Microsoft published a press release containing Banjo-Kazooie's release date, saying that it would be coming to XBLA "exclusively". It's set to release on November 26th this year, with Banjo-Tooie coming to the service early next year. Strangely enough, not long after, the press release was removed. Perhaps it was not for the public eye yet? Oh well, too late now!
Certain other "exclusive" games in the past went on to be released on other consoles, so of course there's always a chance Banjo-Kazooie might be released on the Virtual Console, but for now, things aren't looking very good!