Hi, I'm the guy who created
http://streetunity.org/
Its quite possible to use Asterisk for sending and receiving SMS via chan_dongle and it also has the additional use as a SIP to GSM gateway, only snag being a bit of lag due to transcoding audio on a relatively slow CPU. I'm sure there are ways of improving this, perhaps by using an all-gsm codec-chain...
I decided however, to use SMS Server Tools 3, which can be downloaded as source and will happily compile under Debian armhf, provided you have the right dev files installed. This is mature software which has been actively developed for years, as opposed to chan_dongle and chan_mobile (the latter of which i have long since abandoned)
SST3 will talk to any modem/phone/dongle which supports AT commands. It does not support voice, but in my tests it will happily run alongside Asterisk on my Pi.
I'm currently playing around with things like SIP SIMPLE to SMS and others, if you check out my site you will see some of my work.
Latest project is using the Pi as a Captive Portal with CoovaChilli, DD-WRT (for the VLAN routing) and some old DSL APs to provide plenty of wireless connectivity.. Always good to recycle old routers, and also free advertising for my stuff to the people who use the wifi

Win!
I am starting to document my work on my blog, so stay tuned...