I've actually never thought about it much either way. I always just assumed that the people saying it was were the typical conspiracy theory kooks. I do remember someone on an old forum trying to prove legitimately that one of the buildings couldn't collapse the way it did unless some other type of demolition was set off inside of it but someone who knew a lot about Physics came in and explained how it was possible.
Personally, I don't think the government would actually be so cold-hearted (and I know I'm giving the government a lot of credit there) as to massacre several of it's own people just to justify invading another country when they could easily thought of another reason.