The receiver is no issue. I already can hook up my PDA via Wifi to my home webserver, which hosts up my webcam feeds. The issue here is the transmitter: wifi-embedded devices ain’t cheap. Its cheaper to buy a webcam + used computer + wifi adapter than it is to buy an independant wifi camera (Dlink make those).

So yes it exists. Just not cheaply. That said, monochromatic analog cameras transmitting over non-wifi radio bands can be cheap, but lacking wifi they defeat the purpose.