Quick and dirty:
This thread, the personal arguments in it aside, got me to thinking about sci-fi mixed with AD&D - but "our" way.
Then I got to thinking "Well, what if you wanted to create a sci-fi game, but not AD&D+Sci-Fi, but use the readily available mechanics of AD&D as your base?"
Well, you're in luck because there are rules for firearms and other high-tech items briefly outlined in the Dungeon Masters Guide, and for even higher tech we've got the energy weapons, healing sprays, grenades, etc. in S3 Expedition to the Barrier Peaks.
So we've got our equipment, methods of understanding it, a base rule system for character generation and combat. What else? Anyone?
Keep or ditch demi-humans? I 'spose you could go the way of WH40k and have stuff like "eldar" and "squats", but I think a Niven-esque "hard" SF with humans as the focus (yes, I know, Kzin, Pierson's Puppeteers, etc.). Plus that way you don't have to muck about with demi-human level limits.
Since it's scifi, throwing psionics back in to the mix (while obviously ditching AD&D magic) would be OK. Tweaks would be required, though. Quasi-medieval weaponry would be a must for those "primitive" worlds. There's Monster Manual, Fiend Folio and Monster Manual II monsters galore that fit the sci-fi mold quite nicely, and a lot of them are munificent (the bird-men in the Fiend Folio spring to mind).
What else could we do here?