Elgato's EyeTV app requires you register your Mac to its free My EyeTV service in order to tap into broadcasts outside the house, but use it when your iPhone has only a 3G connection and you'll see an error pop up, warning that "Live TV playback requires a Wi-Fi network connection."
Tap the OK button, and the app will act as if it can't receive broadcasts. However, tap the text of the warning message instead, and the Eye TV app will stream live TV over a 3G connection.
The report notes that the feature almost certainly went unnoticed by Apple's application reviewers and it is unclear how the company will react to its discovery.
Update: The easter egg was apparently first publicly disclosed nearly two weeks ago, just days after the application's release, and Apple has apparently yet to take any action in response to the issue.