Quoted from the Official Banjo Blog:
"The classic Banjo-Kazooie of yore has now been released on Xbox LIVE Arcade and can be downloaded by gamers worldwide as of... now. It can be yours for 1200 Notes... er, I mean MS points.
We've also released a title update for the game that resolves a few of the issues experienced by those lucky, lucky people who have been playing B-K classic until now."
Read all the fixes after the jump