Anyway, my questions are:
1) Besides the obvious (no dirty words or sexual situations), what do you think the boundaries are on content? And, how do those change depending on age groups?

2) Do you think any Sci Fi authors are particularly good or bad at this. I enjoy Andre Norton's novels regardless of the fact they were sold as "juvenile fiction" for the most part, so I'd include her in the "good" category. I'm not sure what a "bad" author of juvenile fiction would be, but I was thinking more along the lines of who's books would you not want younger teens to read. Maybe not Philip Jose Farmer, for example?

3) Isn't most Science Fiction already kind of aimed at a pre-adult market? Is that who buys this stuff? Is there such a thing as an Adult Sci Fi market? ala Kilgore Trout?

Anyone have good resources on how to write for this market on purpose?

Your point one is NOT obvious.
"1) Besides the obvious (no dirty words or sexual situations)"
Plenty of YA books have a few choice words in extreme situations and many have sexual tension and off-stage sex. Read some Tamora Pierce or Susan Dexter to get a feel for what's allowed nowadays.

"2) Do you think any Sci Fi authors are particularly good or bad at this."

Good ones include the above two authors plus Diane Wynne Jones, Phillip Pullman, Madeleine L'Engle, and Diane Duane. Have you read the Harry Potter novels (J. K. Rowling) and the Redwall series (Brian Jacques)? Wonderful stuff. Lois Lowry's The Giver won many awards, but it didn't hold my interest. All the Star Trek novels ever written are YA, aren't they? Oh, and Piers Anthony when he's trying to be nice.

"I'm not sure what a "bad" author of juvenile
fiction would be, but I was thinking more along the lines of who's books would you not want younger teens to read."

The golden age of science fiction is 12.--David Hartwell. IMHO, teens can read just about anything SF and be O.K. with it.

"3) Isn't most Science Fiction already kind of aimed at a pre-adult market? Is that who buys this stuff? Is there such a thing as an Adult Sci Fi market?"

Anyone with a wide open sense of wonder likes this stuff. That includes the young, but mostly, they aren't the ones buying it.

There IS an adult (rated X) market for SF (was that your question?) It is called SF erotica, as in: anything published by Circlet Press. The writing is strictly meant to excite; SF/F elements may have a tacked-on feel.

"Anyone have good resources on how to write for this market on purpose?"

Some ideas:
Choose your character's ages to be slightly above the age of the teens in your target audience.

Get rid of parents/advisors/guardians so that the young adult characters can have the power to make their own decisions.

Keep it short, but don't talk down.

Check out these sites:

Links to Sites of Interest for Authors and Illustrators of Youth Literature http://www.scils.rutgers.edu/special/kay/council3.html

Children's Writing Resource Center http://www.write4kids.com/index.html

Aaron Shepard's Kidwriter Page http://www.aaronshep.com/kidwriter/

Good luck!

This being Hatrack river, I feel it's necessary to say that Mr. Card has touched on writing for young audiences. His advice is
pretty much the same as Diane, that being...

Youthful characters, not grown ups.

Don't talk down the reader.

And I'll add:

Should have some element of wonder or adventure that appeals to teens OR some problem that teens often must deal with.

It's hard to say what is YA and what ain't. It's really only a label, but I've read YA that I found excellent and not trite or teeny-bopperish at all. In fact, sometimes the YA market is the perfect place to tackle tough, real life social issues because it will be that much more effective and
controversial. But YA's main function is to serve the publisher by steering adults away from that section to buy books by the latest, greatest adult writers, and to have an excuse to tell parents when they ask what kind of sick book they found their kid reading.. "Well, all are youth books are designated YA..."

That being said, the YA market is just that-- a market, and the people who read YA are just as smart and just as willing to read about anything the adult market will publish.

as far as things changing by age group, if you're looking at writing for teens(instead of pre-teen) i think most teens(depending on reading ability) like sf-fantasy stuff for adults. if you -try- to write for them you are much more likely to talk down to them(which is the absolute worst thing you can do)