David, if your iPhone is unlocked, good short-term options for Canadian cell service include purchasing a T-Mobile prepaid SIM before you leave the US or a 7-11 SpeakOut SIM once you arrive in Canada.
Some T-Mobile US prepaid plans include roaming in Canada, or offer it as an option. You'd be looking at USD 40-50 for one month. You can order a T-Mobile prepaid SIM card from T-Mobile's US Web site. You can add money online or by phone. You can select most prepaid plans online, but some plans and options require calling in — as does making a new plan active immediately, rather than on a monthly anniversary. Be sure to mark the plan for non-renewal, so that your balance won't be drained. After your plan ends, the SIM card remains active in the US (but not for roaming in Canada), with small per-month and per-minute charges.
In Canada, 7-11 sponsors a popular prepaid cell service called SpeakOut. You can buy their prepaid SIM card at any 7-11 store in Canada, including the handy location on the arrivals level of Pearson Airport in Toronto. (Exception: SpeakOut does not operate in the Province of Québec.) You'd be looking at CAD 40-50 for one month. Because the SpeakOut Web site only accepts credit cards with Canadian billing addresses, Americans have to buy "top-ups" at Canadian 7-11 stores (where US credit cards work fine). The top-up is a long cash register tape with a code that you redeem on the Web site. After topping up, you choose a plan. Be sure to mark the plan for non-renewal, so that your balance won't be drained. After your plan ends, the SIM card remains active, with a per-minute charge for calls. SpeakOut never works in the US.
Your iPhone is unlocked if you bought it from Apple and paid the full, retail price, or if you bought it from carrier, kept your service active for the required time period, and then requested that the carrier unlock your phone. If your iPhone is still locked, you will not be able to switch the SIM card.