Copper heatbed?

Has anyone tried a sheet of artist's etching copper over a silicone heater as a printbed? Any problems?

Isn't that stuff pretty thin? I would expect it to warp unevenly when heated. If you sandwich a heater and a plate the plate needs to be stiff enough to not bend due to the different coefficient of expansion. While the silicone heater might not giv emuch resistence, the copper sheet bends really easy.
Also the thinner it is the more uneven the heat distribution is.

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The bed plate needs to be flat, rigid, and thermally conductive. If you can get copper that is all those, go for it. However, in many printers the bed moves in the Y axis, so having lower mass would be better, too. Machining copper can be quite difficult. As an alternative, cast aluminum is easy to machine, lower in mass, almost as thermally conductive, and readily available for much lower cost than copper. If you have some sort of scientific application that requires even heating to within 0.01C over each cm of the bed, maybe not, but for a 3D printer that prints plastic it is fine.

Here's a thermal image of a cast tooling plate bed on a Taz printer. Note the temperature variation (or lack thereof):

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]