Kimberly, I wish I had an answer for you, because I have the same problem (I think). I can't print duplex from ANY Adobe product (so far). I have a new HP Color Laserjet CP1518, downloaded and installed the latest firmware and postscript driver from HP. Windows XP is at Service Pack 3. I'm running Adobe CS2--I read in another forum that Adobe Tech Support won't talk to me if I don't upgrade to CS3, but I bet they'll just say they can't code to the idiosyncrasies of ever printer and manufacturer. Anyway, My printer allows manual duplexing, which is slightly more convenient than just doing it the old-fashioned way. It works fine with Word documents, and with web pages from Internet Explorer, and with FileMaker Pro databases, printing 2-sided as advertised. But it just ignores the duplex setting for PDFs (ones I've exported from InDesign and received from other sources) and InDesign documents.
====> Do you have any other Adobe products? Can you 2-sided print any documents made by any of those other products?