Piling on here a bit
I too am a big fan of Pandora, but besides the business agreement needed, this also may not be trivial for Roku and Pandora from a technical point of view either.
Pandora is Flash-based, and downloads music files to local cache, so it's not just as easy as getting Soundbridge to accept a radio stream.
Not quite sure how Slim devices pulled it off, but I'm guessing that through the Squeeze Network, they're able capture the Flash content on their servers and then turn around and stream it to the Squeezebox.
Doesn't work with either a Soundbridge emulating a Squeezebox *or* the SoftSqueeze player running on a PC. And requires a subscription fee, unlike just running Pandora on your computer.