I'm not a a Canadian, but my home in the US is ten miles from the border and I"ve grown up along side it with several Canadians in my immediate family so I know Canada pretty well, but not as good as PhotoJim or Vlad.
Canadian prepaids are nowhere as good as in the US. The Canadian cellphone industry (even though almost all the companies are national) are modeled on a regional model. Most Canadians roam when they get fifty miles from their home and this is reflected in the premarket market as well. I live just outside Windsor Ontario (519 area code). London Ontario is just shy of a two hour drive. On most plans, you are roaming in London. On Rogers and FIDO, you have to do a large recharge to get a year of service.
The best Canadian SIM I've found is the 7-11 Canada one. You buy it with a junk phone and pitch the phone. Locals calls are about $0.20 a minute. Internal Canadian calls and roaming calls are $0.30 a minute.
Frankly, it looks like the CDMA side is a little better in Canada with respect to data, but buying a new phone for a prepaid trip usually is a bad investment. Telus, Solo (an MVNO on Bell), and Virgin have some creative options. Bell Mobility has a prepaid with free incoming calls in your local area and free calls after 6pm if you pay a dollar a day:
http://www.bell.ca/shopping/PrsShpWl...reLanding.page
Virgin Canada has unlimited e-mail for $3 per month on a phone if you buy an LG Rumor. $7 more a month gets you unlimited mobile browsing.
Koodomobile also has some interesting CDMA stuff (
http://www.koodomobile.com/en/on/index.shtml). They are not prepaid, but if you pay full freight for the phone, you can go month to month. Koodo rides on the Telus Mobility network. There does not appear to be any tethering options.
I haven't found great data solutions for Canada. If you are on ATT contract, you now buy 50 megs in Canada for about $60. It is better than you can do on the prepaid side. On Rogers, you can add a smartphone mail option for $15 which gives you two megs or so of non-email data.
Even with a contract Rogers phone (I traded one phone of my family plan to a Cannuck relative for one on his Rogers plan), I can't add serious data on a month to month basis. There is a good deal on 4 gigs of data for $60, but I have do a three year commitment just to add the data. [I was wrong, see PhotoJim's post below which says that the deal is 6 gigs for $30, but with a 3 year contract]
I've also look for companies that rent Canadian data cards and have not succeeded.