Here's my humble opinion:
As a separate system Spelljammer may have worked. Flying ship piracy in three dimensions could work nicely in a Space: 1889 kind of way(as noted above). It did have many original ideas that creatively tied in all of the standard D&D aspects into a space-based game. Dungeons, treasure, monsters and magic were all included straight from the original game. Unfortunately, this tie-in to D&D is also Spelljammer's greatest weakness. While D&D could be construed as a plausible medieval fantasy world, Spelljammer was completely implausible. Besides being difficult to relate to it seemed to make fantasy and sci-fi (of which it was a hybrid) appear both silly and comical.
Wizards (or was it TSR still?) redeemed themselves with the release of Alternity, a much better game firmly rooted in hard core science fiction tales. Spelljammer is best left to its own small group of fans, like many of the other smaller settings. Dedicated fans who band together seem to have a better grasp of what is best about their particular game, anyways. Spelljammer fanatics should be the ones who make the new materials. Maybe what they will come up with could prove the game's real worth.
So what are we left with for Space in most D&D fantasy worlds? In first edition some modules dropped in spaceship crashes to have a cross-over feel without removing the fantasy element. 2nd Edition, however, opened up many elements traditionally belonging to science fiction. Time travel, spelljammer space travel, and dimension hopping all seemed to sell poorly.
3rd edition, I 'd suggest trying to be either historically accurate or as close to myth as possible. Maybe we're all closed in by spheres.