Open Preferences > Advanced and click “Show Develop menu in menu bar”.
In the Develop menu, select User Agent and hit “Safari 4.1″.
Enjoy with Netflix now.
Watch some Netflix!
Of course, you'll have to switch it back to use any Safari 5-optimized sites that check for the latest version of Apple's browser, but that's not too hard -- and like we said, we're guessing the Netflix crew will have this sorted in no time, so you shouldn't need to worry about this in the future.