I've been faced with a similar problem: I ended up on a weekly trip to Nebraska for work, and found that my Canadian Rogers phone didn't roam here. So, I bought a Cingular 8125 PDA with data access. I now have phone service and unlimited data on a US plan. The unlimited data come in real handy for accessing email on the road, IM'ing with friends, and surfing the web.

As for Canada, when I'm back home I just pop in my Rogers SIM and use my local Vancouver phone. I have a voice-only plan as I don't really need email, msn and web surfing when at home. By the way, the Cingular 8125 can be easily unlocked with readily available downloads from the internet.

Total cost:
$35 USD montly for voice and data (+ Canada calling plan at 19 cents/min)
$25 CDN monthly for voice