Just in case anyone else has an Rpi900 http://www.rpi900.com
I've hacked the packages to work under Raspian, the supplied packages were for Arch which was not for me.
I get a sustained 143 kB/s (using FTP) over a ppp link on the bench, next step is some weatherproof packaging and range test. Impressed so far.
I'll do a HOWTO at some point if anyone else is interested, once I've tidied up my ugly hacks.
Cheers,
Bob