So I've got a pair of Digi XT09-SI 9 Xtend modules. I'm trying to get them communicating with a base station and an APM in a plane. I first tried the default baud rate, 57600, and range-testing the radios by jumpering RX and TX on the remote end and echoing data back to myself. I got 99% success rate on that. I then tried interfacing to the telemetry port on the oilpan with my FDTI adaptor (an arduino duemilanove with no firmware) at 57600. Again, great success.
The problems started when I tried to communicate via the radios. I could sometimes get all the way through reading the parameters, but only rarely, before the connection dropped out and "Not a 1280... Not a 2560" was written to console. I've dropped the baud rate down to 9600 in the modules and on SERIAL3_BAUD in the APM firmware and now I can maintain a connection indefinitely, but there are still many dropped packets. I am about 50% successful in downloading a set of 22 waypoints without it bombing out with the error (displayed in a popup dialog box, not the console):
Error : System.Exception: Timeout on read - setWP