I've been mobile for the past year, so just a 17" MBP (mid-2010), 8/500. I'm going to upgrade it to 512GB SSD + 1TB rotating HD soon. Right now I carry 5 x 500GB hard drives for photos (and backup), music, and 2 x data backups. The critical component: UltimateEars 10vi headphones (upgrade from Etymotics ER4S).
I usually have a high-spec Linux desktop with 3 x 24" portrait-mode U2410 IPS panels, and a 24-30" second monitor for the laptop, at my desk. I'll probably try some combination of Mac Pro and iMac (at home or office) next, but I think I'll keep the linux desktop too. Critical component: Beyerdynamic DT770pro sealed headphones (office), Sennheiser HD650 open headphones (home). May upgrade to HD800 at some point, or speakers.
I need to be able to do office automation and image editing (and maybe now iPhone dev?) (mac), lots of sysadmin/text editing/dev (mac or linux), and to also have a way to keep "work" and "fun" stuff on separate machines at the same desk. I also often need to have "machine under test" (embedded, server prototype, etc.) separate, and also sometimes need to run one or more machines on an airgapped network.
This leads to wanting 2 x 20A power feeds to my desk, and a good air conditioner. A noise-isolated enclosure would be an awesome upgrade (or a nearby office with some long DVI/keyboard/USB cables and a remote power cycler...)
$250 armless "size C" Aeron at home; probably will get an Embody or something for the office.
Also need to get an office: planning to use a shared office space in Mountain View for Q1-Q2 2011.