Actually, it very well may *be* a separate app. We'll see: my sense is that the ideal UI for a mobile screen is a tad different from that for a large one. Probably sharing most of the same underlying Javascript (it's a fairly sophisticated system), but with a different UI skin on top of that. I've been pondering that, as well as the possibility of highly tuned versions for iPhone and Android, as I think about the API model. Regardless, mobile is definitely in the cards -- the only limitation is my limited number of coding hours.

(There's also an open question of whether to interface with text messaging, a concept I have a real love/hate relationship with.)

XHTML plus CSS: maybe. So far, the model is largely built around a smart client, which I've focused on largely because I can optimize it really, really well, producing a system that is highly responsive from a user perspective *and* doesn't load the server too badly. There will likely be a dumb-client version eventually, but I'm not quite assuming that yet...