I was looking around today and i found this "Water alarm kit". Now if this thing works as well as i hope, couldn't you put it in your water cooled case and if at time something leeks this thing would go off and warn you. This probably won't work but it would be cool.
http://www.allelectronics.com/cgi-bin/category.cgi?category=305&item=MK-108&type=store
http://www.allelectronics.com/cgi-bin/category.cgi?category=305&item=MK-108&type=store