There's not a lot of info on SSBB yet, but here's the gist of the information we have:

It's being developed by HAL Laboratories, and Nintendo have contracted Mr. Masahiro Sakurai (who'd since left Nintendo), director of Smash Bros Melee, to direct this sequel as well.

It will, at least primarily, be controlled using classic gamepad/analog stick control settings, not the motion-sensing control of the Wii remote.

Confirmed new characters are: Meta Knight (from the Kirby series), Pit (from Kid Icarus), Wario, Zero Suit Samus (from Metroid Zero Mission), and Solid Snake (from Metal Gear Solid).  Other secret characters are confirmed as existing, which may possibly include more characters from other companies' games.  Our own speculation over their possible identities include Sonic the Hedgehog, Little Mac (from Punch Out), Pac-Man, Simon Belmont (from Castlevania), Daisy and Toad (from the Mario series), and Mega Man.

Here's a link to the trailer for the game, with no logos attached (many of the ones available on the internet have added their sites logo as a watermark).

And finally, here are stills from that video.  You can click to enlarge them.