From his first appearance on the show, I was not convinced that Henry Gale was one of the Others. I still am not convinced. Yes, he has made a big mistake by lying to everyone about who he is, but there is one key factor that still has me thinking he is someone else -
Both Ethan and Goodwin (?), the others we have seen, had one thing in common, that they both said they came from Canada. Now, I could be totally wrong on this one, but it seems to me that it is just too much of a coincidence that they both say they are from Canada.
Also, we have not been introduced to any other characters, regular or not, that said they were from Canada. (Yes, Kate say she was at one point, but I seem to remember it being part of one of her cons.)I'm getting a feeling that Canada is taboo, or possibly the generic cover-up for the Others.
"One morning, I shot an elephant in my pyjamas. How he got into my pyjamas, I'll never know."
PS: I am also Canadian, and I kinda like the fact that our country could be taboo.