Here is my take on the origin of the Canadian "eh?"
The French keyboard layout.
When you want to type in French, the easiest way is to switch to the French keyboard layout, instead of using the Alt + 0XXX codes for the accented vowels and the c-cédille. However, once you switch to the French keyboard layout, some other keys, such as the forward slash (/), become harder to find, so we Canadians in the French speaking provinces often switch back and forth between the English and the French layouts. Sometimes, though, we forget to switch back from the French layout to the English layout. And this is what happens when you ask a question in English, while typing with the French keyboard layout:
"So you want to go to China Town tonightÉ"
You see that "É" at the end of the sentence? That's what happens when you try to type out the question mark (Shift + /). In the French keyboard layout, that key combination gives you the e accent aigu instead of the question mark.
The sentence ends up being read as "So you want to go to China Town tonight, eh?"
And that's how the Canadian "eh?" was born. :)