I flew into Seoul on a one-way ticket and Australian passport in November 2012 with no problems at all.
I would expect most nations to treat Australians and Canadians in a pretty similar manner.
In my case I did have onward ticket, to Australia seven weeks later. But when I arrived at Incheon International Airport nobody asked to see any such documentation. Korea is pretty cool with such things, at least for people from "safe countries" like ours.
You should get the same three months that I always get in Korea. South Korea is totally open so you can travel where you want when you want, you don't have to register or anything like that.
There is a possibility you could be asked about onward travel in Canada when check in with the airline. It is the airline's responsibility to pay for your flight back when the country refuses to let you in (though the airline will probably try to make you pay for it too). In my case I boarded the plane in Istanbul and was not asked about this though I did have proof of my next trip handy if they were to ask. Now on my current trip I was asked when checking in for my flight from Sydney to Malaysia. They were happy with the $10 train ticket to Singapore I bought on the Internet for just such an eventuality.