Site seems to be having trouble with the traffic though.
Also, I'm trying to envision the slice of the world that simultaneously cares about an "open source" cell phone, but doesn't want a full featured smart phone. Maybe people like that exist, but I certainly don't know any of them (I know that doesn't prove anything, just saying...)
One application I think would be pretty cool for this would be some kind of remote monitoring, or remote controlled device - if you could get an SMS plan with that shield and then hook up sensors/actuators to the arduino, you could potentially get data off it via SMS, or control it remotely via SMS. That would be cool.
Full Disclosure: I know one the founders. Amazing guys working on some very cool things.