I live in Minnesota and have mobile service through AT&T. My iPhone 4S has been unlocked by AT&T.
While visiting Montreal I activated my mother's SNAPFON and my AT&T iPhone 4S with Canada's Rogers Wireless. There are clearly right and wrong ways to do this. I'll share what I learned and update this if I discover a better path.
I chose Rogers because their spectrum is compatible with GSM phones, including AT&T unlocked iPhones. They are the largest mobile carrier in Canada, and even more beloved than our AT&T. I'll outline what I did, then provide some additional detail.
- Decide what PAYGO plan you want. We had two choices - By the Day and By the Minute. By the Day costs $1/day but usage fees are lower. By the Minute is more per minute but there's no daily charge. It's not clear you can change plans. I should have gotten By the Minute for our needs, but ended up buy the By the Day plan -- which will burn through money faster. Surprisingly, you can change plans at any time by using *611 (below).
- Decide what 4 digit PIN you want and what area code you want.
- Acquire SIM cards (micro or full). I picked up 2 iPhone Mini-SIM and 1 GSM full size SIM from a Rogers store at Pte Claire mall. They were free.
- Buy vouchers: $20 (expire 30 days) or $100 (expire 365 days). 
- Activate phone by one of three methods: online (don't ), by phone (need another phone), at Roger's Shop or at some resellers. Tell Rogers which plan you want.
- If you want data you have to add that feature separately. Same for voice mail, etc. See update below; I ended up adding 100MB of date from the Rogers PayGo web site.
- Place and receive at least one call 
- Being a geeky kind of guy I picked up the SIM and thought I'd do the rest online. I'm still twitching from that experience. After I'd walked through various issues, including going out and buying a voucher, I got a message apologizing that my phone couldn't be activated but they'd maybe send me an email. Someday. That came with a phone number for help, which I called and the valiant support person somehow figured out a workaround. Look, just don't do this.
- I strongly recommend having the phone activated at the place that sells you the SIM, even if they charge something like $10 for the privilege. Typically you''d but the $20 voucher and, I think, the all day pay go plan. Don't get anything else - no voicemail, nothing. Certainly don't give Rogers your credit card number. If you give them an email address make sure it's a spam address. When I activated (by phone, because I wasn't smart enough) they said they'd use a "generic" address for me and didn't care about email and phone.
- Once you get a Paygo number it's good for 6 months with no use. After that you lose the number and have to get a new SIM. 
- I bought my mother a $100 voucher and used that to activate. I bought an additional $20 voucher to take back to the US. If she runs low I can use rogers.com/paygo with that voucher to get her emergency minutes.
- Each month money is deducted for "911 service" and for any other benefits you sign up for.
- *225: text balance
- *868: enter PIN if requested then enter voucher number to add minutes.
- *611: automated assistance, includes adding minutes, changing plans -- but all voice recognition. They hide the options for the cheap plans, but I was able to change my mother from 'Any Day' to 'By the Minute'. HOWEVER, I was unable to add/modify features on my iPhone. When I entered the 10 digit number Rogers couldn't find the account. (tone problem?)
- fn -
 you can activate the phone and buy time online -- but this needs a Canadian credit card. Also, I'd sooner give my credit card to a crack addict than give it to Rogers. Lastly, from my painful experience, anything done through their web site is very problematic. Just don't do it.  I've seen this with AT&T too. Until you call out once, and perhaps receive one incoming call, the phone may not be truly on the network.
- Why I switched from Rogers Wireless Pay As You Go to 7-Eleven Speak Out Wireless (Canada!) | Peter's Useful Crap
- Nano-SIM redux: Using the iPhone 5 in the UK | ZDNet
As I noted above when I entered my phone number on my iPhone Roger's automated *611 service didn't recognize the account. I don't know if it was a problem with tone interoperation or their accounts. Since by this point I was a bit crazed I decided to try creating an account on www.rogers.com/paygo.
Surprisingly, since I already had a Paygo phone and had already registered, this was fairly painless. I found out what address I'd been given when I phoned in:
The name is my TrueName, but the address is made up. This is evidently the Roger's "generic" address. I can see why drug dealers like PayGo plans.
From the web site I found I could switch from Any Day to By the Minute -- but since my minutes would expire anyway I didn't bother. For me the Any Day plan was ok. I added 100 MB for $10, so now I can use iMessage and do light email or even consult a map if I have to.
I also learned I could add my MOTHER's number to my account, as long as I knew her number and PIN - despite differing names and addresses. I'm not used to this degrees of anonymity; evidently the PayGo world operates on old rules. So this is actually somewhat convenient.
Another reason to visit this site is to adjust marketing preferences -- though it takes a few days for that option to be available. Of course I gave Rogers my spam-only email on yahoo.com.
The data service didn't work. I reviewed the detailed bill summary and it showed only one data transation of 147KB. I don't know if this was due to bug in Rogers system or if my American 4S isn't compatible with Rogers data services. For the moment I'd say this is a voice and text only solution.
Since I first wrote this I setup a Canadian bank account, so I can top up via Interac. Very convenient. I discovered however that, unlike every other prepay service I’ve used, when you ‘Top Up’ Rogers sets the expiration date to 30 days from payment, not 30 days beyond the current expiration. In other words, you should not top up until the very last day.
This is a scummy practice.