Nancy Salomon, a constructing veteran, warned me in an e-mail earlier this year that "if you post your puzzle at cruciverb.com, most editors consider it published." Having said that, a Cruciverb site administrator wrote the following in February 2009:
Will Shortz told me that puzzles discussed on the cruciverb-l discussion list would not be considered "previously published". I beleive [sic] most editors would feel the same way. I have set the Post a Puzzle forum to be open to members only. Guests cannot view. I would therefore think that the same logic would apply.
There's obviously conflicting information on this, so I'd say if you're looking to send a puzzle in for publication, don't post it just to be safe.
That doesn't mean you shouldn't discuss puzzle ideas here -- there are lots of constructors, both amateur and veterans, who can give you some really neat ideas to spice up your grid. You can probably still submit a puzzle if, say, you posted a draft version here, but then made significant changes to it later (i.e. changed your theme entries, or moved the theme entries to another part of the grid, etc). But if what Nancy says is true, then you're probably better off not posting it. She's been around the block on this quite a bit, and she wrote a lot of what you'll see on the Sage Advice
page, so I'd listen to her.
There are, however, ways around this problem. If you're looking to get feedback on your puzzle before you submit it, here's what I'd do:
1. Try beta-testing your puzzle with people you know personally -- friends, family, etc. If they're frequent solvers, all the better. E-mail it to them, or have them print it out and solve it on their own.
2. Get in touch with a professional constructor (not a publisher) and have him or her look over it. You'll get an honest evaluation of your puzzle and likely won't have to worry about "thievery."