I'm also searching for smthg like that / thinking of development

An attempt could be OpenGTS - seems to be bandwith-efficient

http://opengts.sourceforge.net/

dont know if it can run on n900, have not figured out gpsd on n900 yet

a lil experiment of my was using python to derive gps-coordinates and show it in a simple map-website on my n900 (yes i've played around with lighthttp, very funny)

if u want to make it opensource and maybe wont use google you should have a look at openstreetmap