I've been toying with the idea of a science fiction AD&D game for a while after realising that the old Buck Rogers XXVc game would make a great platform for generic sci-fi games. It'd need a bit of tweaking (ie: lose the crappy THACO) but it'd work, I'm sure. I even got in touch with WotC to see who held the license for the game system but they never came back to me. I can only assume they didn't actually know and were too embaressed to admit to it for they are infallible.
David Bezio has been working on a game along those very same lines but he's gone one better - what if the creators of D&D never went down the fantasy route but opted for science fiction, instead? And if it came out in the 1970's, what would it have been like? That's the game he's working on, a retro-scifi RPG in the vein of D&D.
It's a brilliant idea and you can download a playtest version of the game from here from Lulu. It's well worth it. Give it a play and get back to David with some constructive criticism and/or ideas. I'd like to see it take off, and in the meantime I'll still trumpet the benefits of using the Buck Rogers game to play sci-fi D&D.