As an artist/illustrator, I'm often working with multiple, large (1GB+) files when preparing stuff for print. Photoshop eats up 8GB of ram doing just about nothing, so when you try to bulk apply some rendering action, export files, or hell, even use their brush engine with something besides the default tool presets, the current gen macs don't cut it. And this doesn't into account running a Cintiq and another display, or their awful integrated graphics setup, etc, etc
I'm currently looking at gaming Desktops to replace my current setup because those are the only things with enough horsepower anymore. It sucks because I feel like I'm going to have to divide up a lot of the scripted tasks I have to make my workflow more efficient, given the lack of a real Unix environment on Windows.