At first glance, the new radios appeared to be the same that otherwise normally existed inside the game, which simply chirped out a samba version of the game's iconic end-theme song. Only later was it discovered that these new radios each contain a hidden audio file that's transmitted when you carry them to one particular point in each of the game's levels.
Thanks to the Steam forum's overeager detectives, we've already learned that the most perplexing of these pirate transmissions are in fact SSTV encoded photographs -- the same used by shortwave operators to transmit images over the air.
I just hope this means there will be a Portal sequel. Half-Life 2 didn't keep my interest, but Portal is one of the best games I've ever played.