The Netflix app on the PS3 and Xbox 360 is great, but you can't easily change accounts without deleting and reinstalling the app. Thankfully, a hidden menu exists to quickly deauthorise your account, and access a few extra settings.
If you want to log out of your account and login into someone else's on a Xbox 360 or PS3, you just need to get into the debug menu. From the Netflix app's main screen, hit the following buttons on your controller:
Up, up, down, down, left, right, left, right, up, up, up.
This takes you the secret diagnostics page. On the PS3, you're simply able to deauthorise your account so you can select a new one. On the Xbox, you also get access to all types of technical information. This includes AV stats that display the statistics of the video you're watching, a frame rate counter, a log, and more.
Most of them aren't particularly helpful, but it's interesting to see what's going on underneath the hood if you're getting a bad connection.