I agree with Ketchup, and have been a fan of Dell's stuff. I wouldn't mind having a Netbook for it's size, or one of the new thin MacBooks, but that said...
I have used various laptops and equipment for pentesting, as well as everyday use.
I have an oQo model 2, which is great for pocket / concealability, but it's not functional (without a dock or something) for normal everyday use, and therefore, I would NOT ever make it my primary machine (additionally, it's not the speediest, anyway.)
For everyday purposes (both pentesting, as well as presentations to my customers / peers, etc,) I have an HP TouchSmart tx2-1020us tablet. Run's Backtrack nicely, and I have both usb and pcmcia Atheros cards for it for WEP and wireless testing. I like it because: a.) it IS a tablet, making carrying it around easy with it's 'lightweightness,' size and touchscreen, b.) it has a presentation remote, so I can easily plug into a projector and present on the fly, c.) built-in webcam (when in tablet mode, and walking around, I catch a lot of good 'visual' info about a test site, etc. There are other reasons I like it, but overall, just a nice little unit. Certainly, however, it's NOT the most durable, so that's one drawback.
And for DEFCON, I would very likely carry an older laptop around ;-) So long as you can boot Backtrack, has some open USB ports, etc, and it has plenty of memory, etc, just about any laptop is pretty functional these days. So, as Ketchup so appropriately put it: " I think that just about any laptop would work, as long as you are comfortable with it."
~ hayabusa ~
"All men can see these tactics whereby I conquer,
but what none can see is the strategy out of which victory is evolved."
- Sun Tzu, 'The Art of War'
OSCE, OSCP (Former - GPEN, C|EH - both expiring / expired)