AX 25 is great for APRS, if anyone is writing an app though it should have a console mode for accessing BBS's like on the international space station.
Here is a PSK-31 project that looks easy enough to run on a N900. http://www.qsl.net/n1vtn/phaseshift.html
I dont have a working scratchbox but Petittrack is on the same page and worked brilliantly on the Zaurus source there too. http://www.qsl.net/n1vtn/petittrack.html
My netbook / partition is too small to fit scratchbox so I can't compile these myself.
Any thoughts of trying a one off hack/wrapper of the Android Echolink app? http://www.echolink.org/faq_android.htm