1. Yes, you can go to Canada on valid I-20. When I was in school the international advisor told us that we should get a stamp or a signature in our I-20's if we want to go to Canada or Mexico (I believe it's called the AVR, Automatic Visa Revalidation). You might not need that stamp or signature (I can't remember for sure).
Here is a link to CBP PDF document that describes this: http://www.cbp.gov/linkhandler/cgov/travel/id_visa/revalidation.ctt/revalidation.pdf
2. I guess this point is not needed anymore since you can re-enter US on a expired visa. However, there were some recent changes to AVR from what I know. If you want to apply for a new visa in Canada at the US Embassy and you get refusal you CANNOT re-enter US. From what I know a lot of people were doing this so they stopped this.
Thank you jamesbond. The senario of 3rd question happened to my wife. she did not submitted her I-94.
3. What departure are you talking about? from US to Canada you didn't hand out the I-94?
Again, I am not sure, but if you plan on re-entering US from Canada on AVR you have to keep your I-94, so if you kept it - it's a good thing.
But it's better if someone with more knowledge answer you.
Have a good one.
