As dcrofty said, you'll just need a mobile that is tri-band capable and I think most phones made in the last couple of years usually are, but just call your phone carrier and they can double-check your model of phone and confirm. Also depending on carrier, you'll need to call them in advance to enable international roaming on your account. While you're on the phone to them, you may want to enquire into the international call rate! Mine was about $2 a min and approx 50c per sms with Telstra.
On Telstra, my phone worked fine over in Whistler when I visited and roamed with the Rogers network in Canada. I got coverage pretty much everywhere in Whistler, even out the back on top of Flute Bowl
I used my Aus mobile and plan when I visited, but if you're planning on living or staying for a while, you may want to look into getting a pre-paid account and sim with a Canadian telco.