Netflix uses Apple’s FairPlay DRM to prevent piracy. The FairPlay version on iOS 5 is so outdated that Netflix’s license servers will no longer issue decryption keys. Without these keys, even if the video downloads, it will appear as a green or black screen.
This is the safest, most legitimate method to get the Netflix IPA counterpart onto your iOS 5.1.1 device, and it doesn't require jailbreaking. It relies on the App Store’s "last compatible version" feature. Here is the step-by-step process: netflix ipa ios 511