Whilst some people don't seem to think so, Chris Heathcote certainly does. Chris has written a Backpack client for any S60 powered smart phone, using Nokia's S60 Python. Ok, it's only a prototype so far, but it's certainly an interesting development. I assume it's talking to the Backpack api. I'll have to ask Chris for a copy to play with, I've shyed away from using any of 37 signals product's because they're so desk-bound, despite them looking rather interesting.
It's an obvious move, so what's next? Campfire? I'd like to echo Russ's thoughts that anyone planning a web based product in this day and age is crazy to ignore mobile. Should mobile be treated as a second-class citizen? The numbers of mobile devices compared to desktop devices certainly make compelling evidence that for many applications mobile should be the primary client.
Update: Chris has asked me to clarify that he didn't write it, he actually managed the project. He's sent me a copy and it's jolly nice so far (hence the screenshots).