Watchismo dug up some stills from Aliens that show Ripley’s watch in detail. He then found the exact model, a unique Seiko Speedmaster designed by Giorgio Giugiaro (the guy who designed the Lotus Esprit, interestingly enough) in 1985. The watch has a weird assymetrical case with pushers on the top and bottom of that little square bit there. It was a fairly limited run and looks pretty painful to wear. Ripley, however, seemed fine with it.
Update – Sorry, it was Aliens not Alien.