I don't know about these days, but I remember 147.560 being used for TCP/IP packet radio networks a few years back, at least here in the Detroit area. It seems like 147.555 might have the potential to interfere.
I'd vote for 146.550 as a primary and 146.490 as a back-up. It seems to me like 146.520 would be a good frequency to monitor or call CQ for general QSOs instead of something more specific, like geocaching. I'm not saying 146.520 SHOULDN'T be used, but it seems less appropriate.
Someone else brought up the idea of using UHF. That'd be OK, but VHF covers a bit more ground in hilly or wooded terrain. UHF would be better suited to downtown areas with lots of buildings and large obstructions. How about 446.550 as a primary UHF frequency? We'd have to listen around and see just who has their crossband repeater inputs/outputs there, first, before we commit to any one frequency. Those things are popular in that frequency range.
FWIW, I really think this is a great idea. Since this has been a long-running thread, has anyone had any luck finding any other cachers on the radio? What frequency/ies did you use?