CNet reports that Google has confirmed that they are using an undocumented API in their Google voice-search application. Google's mobile application takes advantage of the iPhone's proximity sensor to start "listening" for a new voice search. While a useful and unique feature, it technically runs afoul of the iPhone SDK's rules that prevent the use of undocumented APIs.
Under the original terms of the SDK, however, applications using such techniques were not supposed to make it through to the App Store. As a result, other developers who played strictly by the SDK rules would not have felt it possible to create an application that duplicated Google's voice prompt using the proximity sensor, whereas those who had the resources to quickly rewrite anything that ran afoul of the App Store gatekeepers could push ahead and test Apple's limits.
Developers willing and able to sneak into the App Store using such techniques could have an advantage over their competitors. It's not entirely clear if Google's usage was officially authorized by Apple or if it was just missed.
App Store Link: Google Mobile Application, Free