Myriam Joire braving the lines) that if you buy an iPhone 4 in a Canadian carrier's store today, you leave with it locked to that carrier -- despite Apple's trumpeting that you can buy it SIM-free. It appears as though the activation process might be responsible for causing the carrier lock -- either that, or the carriers' subsidized models are shipped locked, unlike the full-price devices you can buy directly from Apple. Either way, it's a bummer, and it's certainly something to take into consideration before you buy.
Update: We've now confirmed that the lock is happening at the time the phone is activated -- in other words, it begins life carrier-agnostic. Pretty wild stuff.
Update 2: Tipster Mika G. tells us that unlocked phones purchased directly from Apple do not lock upon carrier activation, which jibes with Apple's wording that "you can change carriers at any time." Carrier-purchased phones, however, definitely do lock.