It's a bit limited at the moment, but a full release and official word from Netflix are expected soon.
At the end of last week, there was a sort of false alarm in regards to Netflix HD streaming. It was unclear at the time whether it was just a misunderstanding or if it was something else entirely. As it turns out, Netflix has been slowly but surely rolling out high definition streaming to computers.
There hasn't been an official announcement yet, but thankfully, it's all starting to shake out. If you go to the instant streaming menu, you'll notice an HD selection in the genre dropdown. It's something that's been there for a while, but there's a new button in the interface.
Once you get to the HD genre, you'll see the options "Gallery View," "Sortable List," and the new options "Show All HD," and "Show HD for PC/Mac." At least, you should. Some users are still reporting that the sort option isn't available for them.
Don't worry if you're not seeing it yet. It seems like it's being rolled out pretty slowly, perhaps to test the servers. We expect to hear something official from Netflix within the week.