Hi
I am a great fan of OSM and tend to adopt a Borg style approach of assimilating any Garmin device that comes past me. High end GPS like the Garmin 62s is dead easy to get OSM installed and with the huge memory you can swing Contour 5 metres on them along with normal Garmin maps. Fine for those of us with deep pockets but not so much fun for cheaper stuff.
Garmin maps are stored on img files. The Garmin 62s supports multiple files. Other units like the Fenix supports two. The gmapsupp.img and gmapbmap.img. This means that you can download the img version of OSM from web and copy it straight to the GPS as a simple USB drive. The img can be found on http://www.osmaustralia.org/garmin.php. Usual warning. Always take a copy of the original img file in case you need a way back!
Now the Etrex 10 has a trick to getting OSM on it. Garmin have been typical and nobbled the software not to allow multiple maps. This means that you can not even load Garmin own maps apart from the preloaded basemap. Also the memory is small at 8mb. But by taking an OSM img file and copying it over gmapbmap.img you can get OSM on the unit. Now as mentioned the OSM map needs to be below 8mb in size. For Tassie OSM for the entire state is 2.4MB so quite feasible for some time to come using the Etrex 10 with OSM in Tassie. For larger states the img file would need to custom rolled. No great issue for the techno-geeks but more mucking around than this simple post can cover.
Cheers