There’s a little bit of info here:
http://forums.groundspeak.com/GC/index.php?showtopic=282370&st=50
and elsewhere in the same forum.
Also the gps bit of the yacf forum – there’s a few threads.
Plus http://www.aukadia.net/gps/
– see the second second down about the ‘page’ key.
(The vista/legend hcx seem to be the default device for audaxing – I suspect the trip computer is probably similar, tho’ probably doesn’t allow for different bikes)
Don’t think the mapping source really changes – you still need to either buy them from garmin or use openstreetmaps in some form (see last link)
I can’t quite make my mind up between the larger oregon or the etrex 30 (ie for hrm support) – tho’ even the hcx is tempting at ~120 for a remanufactured unit with normal warranty..
HTH