Depending on how long you have been here, and whether or not you had a job in the months before you left for Korea, you may or may not have to file a tax return.
Fortunately, Canadians are rather present in overseas job markets, meaning that our government has a lot of support for us, which is nice.
I was receiving letters saying that I had to file a tax return for 2006, and poked around until I found the following phone numbers. To put your mind at ease, it's well worth it to call them. They are quite friendly and helpful and will help clarify the whole tax issue for you.
International Tax Services:
613-952-3741 (currently operating until 9pm, Ottawa time. You can also leave a voicemail)
613-952-3502 (The "problems" line - I was able to connect directly to a live person by calling this number when the other one rang busy)
They do accept collect calls as well.
You can download forms and information on your tax status from:
www.cra-arc.gc.caHope this helps!