I had a recent email from John, WB2OSZ regarding using the raspberry pi for APRS and he has developed an application that looks like a very slim APRS beacon and digipeater for aprs called Dire Wolf . I have yet to investigate it further but for all those with an interest head over to Johns website and take a look. Here is the address.
From what i can see the application acts as a soundcard modem and tnc for programs like Xastir and APRSIS32. I think this needs some further investigation