I've been messing with this in pfSense for a several months now off and on. It's a real pain, pretty much every card requires its own custom kernel patch. I have the one Verizon PC card EVDO card I have sort of working (it dials and connects at least). I'll be revisiting this within the next couple months or so.
If I can get a clean solution working, I'll port it to m0n0wall if it's acceptable to Manuel. Long term I hope to use it in a Soekris 4511 running m0n0wall in my vehicle, with an Atheros wireless card. Mobile access point, largely for my iPod Touch.