One thing to keep in mind is power consumption. There is a limit to how much power a PC will be able to use dictated by the power outlet of most homes. There will be a point were they cannot make higher power parts for just this reason. Here is an article at OC that can better explain it than me.
http://www.overclockers.com/articles1412/
Right now PC parts manufactures are using high power consumption parts because they can not because they have to. Eventually they will have to start producing lower power parts. Especially when times come to a point when electricity demands can't meet supply. Remember the brown and black outs in California and WA?
The thing is this power limit is not higher than the point of actually needing water cooling. Also GPU air coolers are just starting to get better. They have a LONG way to go before water cooling is necessary. Check out the R600 heat pipe blower system:
http://www.overclockers.com/articles1411/
Also a top of the line PC vid card costs as much as a PS3. If this is the case I feel many gamers would rather opt to go console making less of a demand for such high power vid cards. Maybe a shift in the way LAN parties work. Instead of computers they just bring their LAN enabled console and a TV. Would be easier to carry around.