I thought about doing that to my truck too, Havent done it yet but i was reading a forum about it on a F**D site and general thoughts of it were it would get hot and you can hear the engine and trans. real good,
heres the link to the site so you can read it
http://www.truckblog.com/forums/topic-7007-ford_bedliner_in_cab_on_floor
If you dont mind the noise then it wouldnt be to bad i think im going to do the roll on liner anyways though