The ThinkPad looks like a fairly nice machine, but it's a little pricey. With as fast as computers change I've always tried to limit my purchase price to around $1200.
Just out of curiousity, what OS would you want?
Screen: You're going to want something that does at least 1680x1050 (so you can see how your layouts work on higher resolution screens).
Hard drive: I'd say 5400rpm. Your disk doesn't need blinding speed. But you'll probably want as big a drive as you can get.
A high-end graphics card would be NICE (as it wouldn't rob RAM from your system) but not necessary.
I'd REALLY recommend a secondary box running VMWare to test various browser configs.
OS: XP 32-bit (unless you find a manufacturer with XP x64 support so you can test IE6 or IE7 in both 32 and 64-bit flavors).
WARNING: Just found this out today working on my company website. XP-64 IE6/7 32-bit isn't QUITE the same as XP-32 IE6/7 32-bit. There are certain instances where the XP-32 bit iterations garfs on certain things all other browsers just render properly. Wish I could tell you exactly why.
Recommend C2D (the faster the better)
RAM: The more the better, but at least 2GB.
Compatibility testing Software: Opera, Firefox 2, Firefox 3, IE6, IE7, Safari, VMWare (with multiple Windows, and a couple Linux instances).
Also, not sure if you're one of the "new breed" who's been told that you mock up an image of the way you want the site to look, then chop it up in Dreamweaver/etc. I REALLY don't recommend doing it that way. Sites built that way tend to have massive display problems from browser to browser, long load times, and really piss-poor page rankings on search sites.
Notepad is your best friend.
Actually laying out the page both semantically and hierarchically will improve your skill and actually decrease the amount of time you use actually building a site.
And while you MAY not normally spend more than $1200 for a laptop, you get what you pay for. If you think you can do decent layout and graphics on a 1440x900 screen....
Take my advice, *splurge* a bit on this laptop. It'll do exactly what you need and will last you a lot longer.