Banjo-Kazooie will be racing alongside Sonic and friends exclusively in the 360 version of Sonic & SEGA All-Stars Racing. It makes me wonder if the Wii and PS3 versions are looking at exclusive guest racers too.
In an old leaked document (SEGA, tighten up your security!), the possibility of console exclusive characters was discussed. Ratchet and Clank were considered for the PS3 version, but that apparently didn't go through (according to the document, anyway). There was a little note that mentioned the possibility of characters from "Rare or Fable universes" (obviously referencing the 360 version, though the document is specifically about SEGA and Sony/SCEA). This leads me to think that SEGA got the OK for Banjo-Kazooie from Rare and Microsoft early on, and they started exploring possibilities for other consoles. Welp, here's hoping Mario is included on the Wii version... though I won't get my hopes up too high.