Taking unlocked iPhone 4 to Canada?
Vancouver is the home territory of Telus, so they're probably your best bet. All three of our major carriers (Rogers, Telus and Bell) support GSM and carry the iPhone 4, so you should be fine.

The only Telus pre-paid data plan in BC appears to be $20 for 250MB, http://www.telusmobility.com/en/BC/prepaid/rates.shtml

Rogers has a $45/month pre-paid, includes 500MB, that looks decent, http://www.rogers.com/web/content/wireless-products/plans