For those interested, I’ve decided to go w/ a Powerbook 12″ (I’ve been lugging around a 1st gen 15″ TiBook for the past year and a half) for my new laptop. I have a PB12″ vs X31 excel spreadsheet outlining listing some of the why-fors. Basically, it comes down to the fact that though the PB12″ gets blown away in the hardware department (it is fast enough to be useful, but barely), OS X’s laptop handling and native *NIX environment makes up for it. The biggest things I miss when I’m on a Mac: TopStyle, Neat Image, WINAMP 2, ACDSee, and Trillian. (dealing w/ CRW files is much more limited on the Mac as well, but the next version of Photoshop will come bundled with Thomas Knoll’s kick ass Adobe RAW plug-in, so I’m not super worried about that)
On the flip-side, useful apps that are Mac only: iLife, AppleScript, Konfabulator, OmniGraffle, Final Cut Pro.
Most of the apps I use all the time: Mozilla, vim, and Photoshop, are on both platforms.