[pizzaboy192]

I've been told that Cricket and MetroPCS will gladly use the phone, but you won't get 3G until you install a provisioning .cab file from either XDA or here, and you won't get MMS unless you remove Arcsoft PowerMobia and install the proper MMS settings files (also available either on xda or here)
Custom ROMs do help the situation, but since most people who use the Fathom also use it as their main phone, I doubt many people have dabbled with removing all carrier branding completely and then adding it in after.
if i had a second one, I would gladly do the testing, as I'd like to find a way to disable the CDMA half completely when on a GSM network (and disable the whole defaulting to Verizon stuff)
I think Sprint also will take the phone if you beg enough, but that's just speculation. I know they've taken other Verizon phones before, so I don't see why not.
No idea about US Cellular, as I don't really know much about them.