I have a couple PoE powered keystone jacks sticking out of a wall in a computer lab. We're going to remove /block the excess ones eventually, but for now I'm worried about somebody plugging a laptop into the jacks -- not because of them being plugged into the network, but because they'll hold me responsible if their machine is fried by the PoE keystone jacks. I'm also kind of curious, but I don't want to test it out without more info.
The PSE (Power Sourcing Equipment) and PD (Powered Device) negotiate to see if the end device is compatible of being powered and at what standard. So plugging in a non PoE device will not damage said device.
Here is a good read about the basics of PoE (Do note though the document is quite old and was written prior to the 802.3at enhancement which modifies many of the issues covered.)