More importantly, how do I get a set for my kids (and me)? Speculators sell them in the vicinity of $100, but I'd rather not reward their hoarding if there's another option. None of the stores on Rebrickable have the full parts list. Does Pick A Brick keep everything in stock?
Gev already answered the question about availability. LEGO Ideas sets are produced in small quantities. Perhaps this will change in the future as LEGO Ideas is now out of beta testing and was brought in-house by LEGO. The Curiosity is not going to be back unfortunately. I just called LEGO about this a couple of days ago.
As far as the Curiosity inventory, BrickLink has the full parts-list here: http://www.bricklink.com/catalogItemInv.asp?S=21104-1. None of the parts are rare or difficult to find. You can use the parts-list linked above, add them to your wanted list on BrickLink, then quiry your wanted list by shop. This will show you which stores have all the parts you need in stock. And you can download the building instructions from LEGO's customer service website here: http://service.lego.com/en-us/buildinginstructions/
I don't recall an official statement, but one story I read said that they made a deal with the original designer to limit production to one run, but that seems almost apocryphal.
As for buying the pieces you need, I would think you'd have better luck on Bricklink.
It's possible that the PaB on S@H might have all the pieces necessary, but that seems truly laborious and pricier than Bricklink.
Note: this is all off the top of my head with zero research behind it, so grain of salt and all that.