Most of them are available on Amazon Instant Video.
Most of them are available for rent (which you mention in passing later, I just wanted to highlight this). If we're comparing Netflix to Amazon's Prime Instant Video, they're largely identical movie lists (anecdotal experience only, no hard data). However, Amazon has an advantage in that it offers other films for rent. Netflix did away with its own advantage of "if you can't stream it, at least you can get it delivered on DVD" (Amazon's advantage is more instantaneous, but doesn't have the catalog of Netflix's DVDs, even for rent). I do wish Netflix had a "you can stream this other stuff for $2-3" but I think their customers would revolt (again).
So for now I'm using both. Check on Netflix (better interface on XBox), then check Amazon Prime (which I have anyway for the shipping), then look for films on Amazon Instant to rent if there's something specific I want to see. We'll see if Netflix can distinguish itself with added content.