I got an email pointing at an interesting site about sculpting
metal by a method analogous to fast-prototyping-in-plastic machines,
but in stainless steel + bronze. See link to exone and description
in the second half of http://www.bathsheba.com/sculpt/process/ .
Links from exone refer to casting cores and molds direct from CAD.
(Of course, for many molds using a CNC machine shop might be better.)

Briefly, the bathsheba page says, "work up the design using CAD
software", then at exone "the design is laid down, one layer at a
time, in stainless-steel powder held in place by a laser-activated
binder", "the whole model is built up," "extra powder is shaken off,
the piece goes into an oven, where heat drives off the binder and
fuses the steel powder", producing a "porous steel part that's about
60% dense", then the amazing part: "the stems are dipped in a crucible
of molten bronze, and capillary action causes the bronze to wick
throughout the piece" and "the end result is a composite metal that's
fully dense, with properties intermediate between steel and bronze.
It can take a polish or a patina, developing either rust (on the steel)
or verdigris (on the bronze)".

-jiw