There seems to be some confusion about the role of the word "eh" in the Canadian vernacular. This word is used in both the general Canadian standard dialect, which is one of the few features which distinguishes this dialect from the Midwestern accent, and also in Atlantic Canada, my own home environment. As a proud Canadian myself, a shameless user of the word in question, and thanks to a smattering of linguistic knowledge, I may be able to clarify this for you. It's not complicated, eh.
An interjection of sorts, "eh" (pronounced /eι/) is probably best described as a collaborative particle. As a particle, it has a variety of grammatical functions beyond the scope of prosodics and inflection. It usually comes at the end of the clause it modifies, which usually constitutes a complete sentence. Sometimes, though, the particle is used directly after an adverb; for example, I wanted to begin this sentence by saying, "Sometimes, though, eh, the particle is used etc."
I call "eh" the "collaborative particle" because it is used to encourage acknowledgment, in the form of response, of what is being said. For example, consider this sentence:
The door was open when I got here.
This, in and of itself, could fulfil any number of roles in speech. It could be the response to a question, a repetition of something said earlier in the dialogue, or simply a flat assertion. But consider, conversely, this sentence:
The door was open when I got here, eh.
To someone familiar with the use of this particle, this sentence has a subtle and more specific meaning than the previous sentence. In this situation, the speaker is stating a fact for the attention of the listener which the speaker believes that the listener doesn't know, or perhaps may have forgotten. The speaker therefore requests that the listener signal that he has received the information, even if it's simply in the context of future remarks. This specific and more complicated linguistic context is communicated by the single syllable, "eh."
The particle also has a couple of other uses. When used with an imperative, it is meant to make the issue more polite, indicating that it's something you might both agree on. It is not usually used with questions, except stock questions like "How are you?" where it is used in conjunction with elaboration to make the question sound more sincere. (In the case of "How are you," the question usually becomes "How are you doing, eh?" or "How's it goin', eh?")
The collaborative particle, although the object of much ridicule from our neighbours to the south and our cousins across the Atlantic, is an important and powerful aspect of Canadian dialect. It permits the terse communication of a great deal of contextual information, and is therefore a powerful and valuable addition to our vocabulary.
My linguistics professor, herself from Newfoundland, told me that Newfies have a different collaborative particle, "wanh" (pronounced /wã/, etymologically derived from "what"). It's used in exactly the same contexts as eh, as well as being used as a response to misheard or misunderstood questions -- or questions you don't want to answer, like ansate was talking about.
Non-Newfie Wife: How the hell did that cow get in our bedroom?
Newfie Husband: Wanh? | 
