Yesterday's update (Mario Circuit) and today's update (Waluigi and Starfy AT's) seem to confirm a little theory that I've had regarding playable characters..
When the Mario Kart stage was first show in video form (the first Brawl video, months ago) all of the racers were Shyguys. I'm going to assume, now that we've had a real update about the MK stage, that all characters from past MK games are either going to be playable, be AT's, or be non-playable NPC's.
These past two updates seem to confirm my theory: we've already got most of the cast of the first two MK games playable (Mario, Luigi, Peach, Yoshi, Bowser, DK, Wario) Toad and DKjr haven't been in SSB yet, but both were trophies in Melee.
Petey Pirahna and ROB the Robot both have been confirmed as NPC's (Petey was in MKDD and ROB was a secret character in MKDS).
Waluigi being an AT seems to wrap things up nicely.
This makes me think how everybody else from the MK games (MKDD and MKDS in particular) are going to be in.. If Toad were playable, they could give him a Toadette costume, just like Peach has a Daisy costume.
EDIT: I forgot that Petey was a hidden character. Whoopsie!