There's no telling how long this will continue to work, but as of right now, Netflix is fully functional on the HTC EVO 4G. See update below.
Netflix apps for Android have been floating around for a while now, but streaming has never worked. Last night, however, XDA's Maptec discovered that after the app he had been using to manage his Netflix queue automatically updated, video streaming started to work.
I don't know if the app will work on all ROMs, but it's working perfectly on Myn's Warm Z TwoPointTwo and various versions of CyanogenMod.
The file, which you'll find below, is wrapped in a .zip but it's not meant to be flashed. Just download the .zip, extract it, move the .apk inside it to your EVO (or email it to yourself), and install it like a normal app.
Then just launch Netflix, sign in, and enjoy! Maptec said the app had to auto-update before it started working, but it worked at first launch for me.[xda-developers] Thanks, @JoshuaAlba!
Update: It was good while it lasted . . . and it didn't last long. Video streaming doesn't work anymore.