According to this document, they're still there.
http://support.apple.com/kb/HT3302
With the aid of a small flashlight, I can very clearly see the moisture sensor in the headphone jack of my 4S. I'll admit, I couldn't find the sensor in the dock connector - it must be in a different place in the 3S.
EDIT: Oops, I'm sorry. I can still see the sensor in the headphone jack of my 3rd gen. iPad, though it is smaller than the sensor in my iPhone. Can't find it in the dock connector.