caveat: photography is just one of many things I do and enjoy, and I'm a certified geek.
I built my own workstation - Intel Core i7 2.93GHz, 12GB RAM, 2TB SATA RAID (8 spindles), two dual DVI Graphics cards, 4 LCD monitors (2-20", 2-24") - runs Ubuntu 9.04 workstation, 64-bit.
For processing large datasets (many hundreds of MB) with tens of thousands of files, there's just nothing like UNIX or Linux and the command line (sed, awk, xargs, etc...).
I run a new iMac 20" (OS X 10.5, C2D 2.66GHz, 4GB RAM) for some work I do with Canon's software suite and various other processing. In fact, I used this just yesterday to upgrade my camera firmware.
I run Vista-64 on my Lenovo ThinkPad W500 (C2D 2.8GHz, 8GB RAM). I find that Vista-64 is a heckuva lot more stable than Vista-32 ever was, and it can use all of the RAM in this laptop.
I run Ubuntu workstation 9.04 64-bit on my old ThinkPad T61p (CD 2.4GHz, 4GB RAM).
I run a dual Xeon 3.06GHz, 16GB RAM, 14TB SATA RAID array (16 spindles) server (Ubuntu server 9.04, 64-bit) as my primary network data store, first level backup storage location for Mac OS X and Windows, and main CPU cruncher. This server houses my x0,000 RAW, x0,000 JPG, and my edited photo collection - it's also my main music server feeding the whole premesis, the main file server for the LAN, and the main Virtual Machine server.
For Windows XP, I find it's much more stable running as a virtual machine in VirtualBox (free and Open Source) on top of Linux - that and it boots completely in like 15 seconds flat. This runs QuickBooks and other utility software that I need.
I run a couple of Thecus n5200a NAS devices (Linux) as nightly and archive disk backup systems (rsync'd via cron). I also have the ability to rsync across the Internet to a remote location for offsite backups.
When I need to get more advanced with my post-processing workflow, then I'll find the product(s) that fit my needs and put them on the O/S that they run most reliably on.
Frankly, I don't know what I would do if I had to choose just one platform.