Generally, all the kinds of software that has smart-quotes in it in Windows has it in OS X as well... all the major document creation software in OS X uses smart-quotes. They're also available at the API level, as I understand it, and have been for a long time.
You don't get smart-quotes in a text box inside MSIE8 or FF3.x window in Windows XP... or in a plain text (i.e. a .txt file that has no formatting features) document or at the command line.
That's true also in every version of Linux and Unix I've used. I'm assuming that's true also in Vista and Windows 7 also (have not used either extensively).
If Snow Leopard expands them beyond implementation in document design software, then it would be offering something I've personally never seen on a personal computer of any kind before....