Well, it runs stock Debian- so I imagine either the freesmartphone.org stack or oFono (both packaged), coupled with a relevant user interface. Really the UI is the hard bit, you can send and receive SMS just by cat-ing stuff to the tty device. Ubuntu has a lot of oFono-based UI stuff that can be stolen, though there's quite a few dependencies. Given how the Pandora community rallied around their platform, I've no doubt that we'll end up with a plug-and-play solution before long.
Also it'll probably be awkward for phone calls without some sort of headset (or just putting up with speakerphone all the time), but I hardly ever make calls anyway and it'll more than make up for that by being awesome at texting.
Also it'll probably be awkward for phone calls without some sort of headset (or just putting up with speakerphone all the time), but I hardly ever make calls anyway and it'll more than make up for that by being awesome at texting.