I see that wifi and xbee are popular options.
From reading on the forums, some people seem to have trouble getting wifi to work.
I need my robot to stay in communication around my quarter acre yard.
I don't mind putting a couple wifi (802.11) repeaters up if need be.
Is there any reason I should not use wifi? Has anyone gotten it to work?
How big is the bluetooth range?
Check in the "hardware" posts or the tutorials. A lot of folks here have used the xbee for extended communication range and it seems to be the best option.
Have a look at Sparkfun Electronics
there are some very good options for 6 mile range and one for 40 miles if needed.