I have seen some issues with TC's sitting in water that is trapped in the removable liners... the floor of the campers rotted out. (silversand recently mentioned it)
I have also seen the screatches and rust dammage from water trapped between the removable liner and the trucks bed...
I was warned of this when i bought my camper... I was told to use the spray in liner and make sure the drain holes at the front of the bed are open.
I had the spray-in Linex bed sprayed in the day I bought my truck new in October of 2002... the bed is still in very good shape and so is the Linex (My camper is loaded 99.99% of the time... that may help by keeping the sun off the Linex, I don't know.)
Note... if the spray-in liners are ever digged up they claim to be easily touched up.