I do that with certain TV shows, like 24 and Lost. I figure I'm going to watch next week anyway, so why spoil anything? I'd do the same with movies trailers if I could, but there's just no way to resist, for a couple of reasons...
1. Films take so long to come out, and a trailer helps keeps me content (at least for a little while).
2. Many movie trailers are just so damn well-made. I think of them as an artform in itself. I can watch a well-put together trailer many, many times.
The new Superman Returns trailer, in my opinion, failed to capture that sense of magic and wonder I was hoping for. The teaser did it wonderfully.