Unlocked iPhone in Canada, later use in US?

YES YOU CAN! thats the whole point of buying unlocked phones, u can just pop in a micro sim from any country and Use it on pay as you go or plans, you choice. Hope this helps.

AT&T doesn't offer PAYG plans anymore for the iPhone or any smart phones. You'll need to sign up for monthly service. You can contact AT&T for further information

Doesnt T-mobile still have a Pre-paid plan? I'm not sure though

I believe they do, however, T-mobile's 3G service is incompatible with the iPhone. You will only get EDGE speeds.

You would need to buy a full price unlocked iPhone 4 from Apple, not from one of the authorized carriers such as Rogers, Telus or Bell as authorized carriers and their resellers sell locked iPhone 4 which means you'll be on a 3 year contract with a buyout ECF ($500.00 Rogers, $600.00 Bell, $820.00 Telus). As for which carrier I would suggest Rogers or it's little company Fido. Reasons for this are simple..cost and coverage. Fido runs on Rogers Nationwide network and when you're not in a HSPA+ (3.5G) coverage area which is rare the iPhone will default back to EDGE. Unlike Telus and Bell who offer HSPA+ but have EVDO in their none 3.5G coverage areas which the iPhone doesn't support. Unlike AT&T both Rogers and Fido can activate you on a Prepaid (Pay as you go) voice plan and add on what's called a Data Day Pass ($2.99/Day for 20 MB or $7.00/week for 125 MB on Smartphones). If you're using data daily for 30 days or more you may want to get a Post-Paid monthly plan to lower cost or just opt to use WIFI if it's available. Information on Rogers Hotspots across Canada here http://your.rogers.com/store/cable/internet/wifi/overview.asp As for the Prepaid option it doesn't require a credit check but does require a valid mailing address. Alternatively you can get a monthly Post Paid plan which requires you to do a credit check (2 pieces of Canadian ID including a Canadian mailing address). If you know the address where you are staying you can find out the network coverage and plans with Rogers here http://www.rogers.com/web/content/wireless_network From experience while you're here in Canada you'll notice a faster and better signal on the iPhone 4 with Rogers/Fido HSPA+ 3.5G (21 Mbps network) compared to AT&T in the USA with their HSPA 3G (7.2 Mbps network)

The iPhone 4 is a quad band GSM phone that supports UMTS and HSPA (formally called HSDPA/HSUPA) 850, 900, 1900, 2100 MHz and EDGE with the same frequencies. An unlocked iPhone 4 would run on T-Mobile HSPA+ 3.5G network as long as the carrier supports activating MicroSIM in their system. As per the information that's available the company opted for HSPA+ 3.5G over GSM running at 2100 MHz which the iPhone 4 supports.

T-Mobile uses different 3G frequencies than AT&T, and the iPhone does not have the necessary frequencies for the T-Mobile HSPA+ or their normal 3G. As previously stated you will only get EDGE speed on the US T-Mobile network with an iPhone.

I stand corrected 🙂 Searching other sites such as Wikipedia I confirmed T-Mobile in the USA is running AWS 1700/2100 MHz for their 3G instead of what other Global GSM carriers run for 3G HSPA or HSPA+ in the 850, 900, 1900 and 2100 MHz spectrum. T-Moble should change their 3G information on their website so as to not cause consumer confusion by saying they are running HSPA+ for their 3G network. I had to search their website to find information for consumers wanting to use an unlocked iPhone on their network to find that the user would only get EDGE, not 3G as their 3G network isn't compatible. Instead of following the upgrade path carriers such as AT&T, Rogers, Fido, Bell and Telus in North America T-Mobile opted to use the little supported AWS for their 3G. I'm surprised the company went this route for 3G. Here in Canada Rogers has the same data speed as T-Mobile at 21 Mbps being upgraded to 42 Mbps but at least here we advertise correctly saying it's HSPA+, not AWS which is what T-Mobile is using for 3G. Looks like they took a wrong gamble thinking Apple would jump on to the unsupported AWS technology with the iPhone 4. It makes me wonder if T-Mobile even attended the annual GSM World Congress where telecommunications companies meet to decide on Global standards.

If I buy an iPhone 4 in the US - from the apple store, without an AT&T contract, can I use it in Canada with a Canadian Provider (ie. Rogers?). I went to an Apple store in San Diego and two of the sales reps told me that it was not possible and an iphone 4 purchased in US can only work with ATT as they are locked. Further - I was told that if I had it unlocked - the warranty would be void. Is this true? I'd like to buy my phone in the US because a. they're available and b. they're much cheaper!

You cannot buy a US iPhone for use in Canada. The US iPhone is carrier locked to ATT and ATT does not offer unlocking. The sale staff was also correct that you'll void the warranty if you hack it.

The reason the iPhone is cheaper in the US is ATT heavily subsidizes the cost of the phone. To get the phone for $200+tax, you would need to agree to a 2 year ATT contract.

I'd like to buy my phone in the US because ... they're much cheaper!

They are cheaper in Canada, but require a three year contract vs two year in US.
<http://www.wireless.att.com/cell-phone-service/packages/iphone-packages.jsp>
<http://www.rogers.com/web/link/wirelessBuyFlow?forwardTo=PhoneThenPlan&productT ype=normal&N=52 114294945848>
Both these are locked iPhones; the Canadian locked to Rogers, the US locked to AT&T.