Apple has been using Liquid Contact Indicators(LCI) in practically all of their devices for as long as I can remember, but it looks like with the iPad 2 they’ve stopped using them.
This makes sense because these sensors have not been all that reliable and subject to a lot of false positives from something as mundane as sweat. In 2009 this was a big news topic that I covered for CNET and I spoke to local Channel 2 news in Houston, Texas about complaints they had received from iPhone users. Even CNN had something to say about it.
It was so easy to trip these sensors that there was even a lawsuit over it.