http://www-esg.lbl.gov/esg/personnel...nsoldering.pdf
Take a look at the indium-bismuth solder. The eutatic point occurs at 78C for a ~79% indium solder. Meaning we could use it to solder a waterblock directly to the CPU die without cooking the CPU (which is typically rated to at least 90C). It also lists a few more exotic solders with even lower melting points.
The pdf mentions typical difficulties in soldering silicon, however it also proposes a solution. Add a thin metallic coating to the silicon die and then solder that.
Does this look viable to anyone else? I'm pretty tempted to go and get some of that solder if I can find it anywhere. The intermolecular bonding in solder should allow for conductivities an order of magnitude greater then at present.