Well, yes and no. Science fiction is a tricky beast, and can more or less be broken down into the sub-genres of "hard" science fiction, cyberpunk science fiction, military science fiction, fantasy science fiction, and media tie-in science fiction (although there are some that don't quite fit in, e.g. Douglas Adams' Hitchhiker's Guide to the Galaxy, which is more or less humorous sci-fi).
Hard science fiction (think Isaac Asimov, Arthur C. Clarke, Larry Niven) has been on the decline since the end of the Golden Age of Science Fiction, i.e. for about half a century at this point, so if you're writing in this area your sales will probably be modest at best. Cyberpunk, military, and fantasy are a different story, though; from what I can tell, those genres have been on the up and up in recent months. (A representative from Borders and the BookScan numbers agree.) This is especially the case for fantasy science fiction, which, in my opinion, enjoys the best of both worlds (think China Miéville's Perdido Street Station).
Now, there is some nay-saying going on, but it largely pertains to the state of the industry as a whole (hint: sales are soft), and insofar as it addresses science fiction sales, there's really no indication as to where the data are coming from, so I think it's safe to say that for now, science fiction is a modest oasis in the unrelenting desert that is book publishing. (+5 metaphor, +2 charisma, +3 constitution.)
Speaking of stats and, by association, D&D, media tie-in sci-fi/fantasy (sort of D&D, but mostly Star Trek and Star Wars) continues to do well. If you're one of the lucky few who's writing those official Lucas Books Star Wars tie-in novels, your sales are secure. But you already knew that.
Finally, what kind of advance can a sci-fi author expect these days?
Once again, it's Tobias Buckell to the rescue. According to him, you can expect a median advance of $5,000, an average of $7,000, and a range of $0 - $20,000. Once again, you'll notice the agented submissions do significantly better than the unagented ones. Agents, people. You need them.
So, in summary:
· Overall sales are still down, but science fiction sales are slightly up!
· Hard science fiction will be a hard sell for you, but everything else should be more or less okay. Your best bet is fantasy/sci fi, I think. (See all previous caveats concerning my advice.)
· According to our man Tobias, you can expect a median advance of $5,000, an average of $7,000, and a range of $0 - $20,000.
To my spaceship, and away!