Notice that the casing of the D200 is now magnesium instead of the
"high impact" plastic of the D100.  This no doubt cost Nikon
considerably more money, but from a marketing standpoint, it made good
sense.  Not all marketing orientation is purely to sell.  Some of it
takes into account perceptions that the photographer  may not have
until some time after they've purchased the product.  This creates
brand loyalty and its obvious that a metal bodied "enthusiast" (the
term used for this level of camera) camera is far more desirable than
a plastic one and is more likely to keep owners from looking longingly
at the competition.  Canon now has only 1 plastic-bodied DSLR, the
Rebel XT.  It has five or six that are metal.  
The absolute truth is that Nikon could have produced the D200
with a plastic body but they chose not to, even though it could have
been brought in at a price point possibly even lower than the 20D.
Look for the D70 to go metal at some point in the future too unless
they drop its price point to lower than it now is.
-Rich