Hey Ben,
Don't forget about structural integrity in all this. A 1mm thick base isn't liable to support the 12-24 lbf of clamping pressure (AMD spec) without excess deformation.
As to removing the heat the same way it goes in, it doesn't really work out that well. It gets in via conduction through a thermal interface of some finite thermal resistance. Interface resistances tend to be small compared to the convection coefficients we can generate. This means that even direct impingement coolers require higher delta-Ts on the water side than the "inlet" from the die. Isolating yourself on an island around the core would also be detrimental. Yeah, you'd force all heat to get out through a small area, but so what? All you're doing is losing out on what small percentage would otherwise escape outside the "island". Given an alternative between a good conductor the size of the die only and another with the same conductance but much larger area, the larger area will do better even with direct impingement.
Oh yeah, and no problemo taking a look at Michael's design, I just didn't have time at the moment I read your post earlier.
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