I do use "thru" when it's casual/personal correspondence, usually when I'm on IM. I don't think I'm using it because I'm lazy though, it's more because I feel that it conveys a sense of closeness to the person because I know that the person won't care anyway.
Which also means that I would never use that in an e-mail that is for business use, or with people that I don't know very well.
Bottom line - I'm fine with it being a new word for casual use, but against the idea of it replacing the original word entirely regardless of usage.