Well, I'm not entirely certain as to what "pre-teen" is. 9-12? But when I was around that age I wasn't really reading books for my age group. Well, except for Sweet Valley High, but it was a guilty pleasure.
If they display an interest in something, let them read it. Regardless of whether it's too advanced for them. Yes, certain themes may be a bit mature (i.e. A Clockwork Orange, American Psycho), but so long as there is no graphic violence, torture, or rape there shouldn't be an issue. Books with political messages should be fine as well, if they catch the references, it can spark an interesting conversation. It's always good when kids think and ask questions.
As far as sci-fi goes specifically, I've always been more of a fantasy chick myself. Loved Dragonlance, it's total fluff but still fun. They even have children's versions of the first trilogy now, although I'm not sure how much easier to read they can make it. It's not exactly difficult reading material to begin with.
Oh! William Gibson. I read Mona Lisa Overdrive, Count Zero, and Neuromancer when I was about 12 and loved it, then explored his other novels. Good stuff. Orson Scott Card is great as well.
http://home.austarnet.com.au/petersykes/topscifi/lists_books_rank1.html
Top 100 Sci-Fi books.
Hell, if they're ambitious throw a copy of Cryptonomicon at them and see what happens. If they don't like it, read it yourself, great book. And it's ridiculously long, perfect for road trips or long plane rides.
Doctor Who is great sci-fi. I haven't picked up any of the books yet, but now that I have a two years or so until the next series I may start reading the novels until I can get my fix.
;)
All else fails, toss them in the sci-fi section of the library and tell them to look around and read. I suppose it all depends on how much they enjoy reading. Some kids just aren't into it. I was very happy on summer vacations when my parents dumped me at the library.