I tried this. roadrunner offers an x amount of free dialup hours to their broadband customers. I used my cell phone as an IR modem on my laptop to dial a local (to the phone) access number (free nationwide roaming/ld). free + free = free.
it's ok for checking email, getting directions, but that's about it. the laptop reported ~33.6 kbps but in reality it's something much lower, even at full signal strength. you might get better speed with GPRS, I didn't try that
I've seen RVs with satellite broadband though not really practical for a car though. if you want reliable wireless on the road, I'd get t-mobile hotspot. it's a pay service, but every starbucks I've seen in the US has it.
