NickPyner:
Great idea, but I still think wireless might be a better bet, it surely involves a lot less digging.
I suspect it depends on many factors. If the Arduinos are battery powered, I could imagine the drain of doing wifi might drastically reduce the amount of time the batteries will last. You probably don't want wifi, which 802.11g is something like 95m best case for access points (according to wikipedia). I don't know if there are Arduino shields that do 803.11n instead of 803.11g which seems to give you more range.