My current laptop (a mid-2007 Macbook, machine model "Macbook 2,1" with the T7200 Core 2 Duo) is starting to age: the edges of the case are splitting (like all early-revision Macbooks of this model), I've yet to replace a broken 'i' key cap, part of the display is slightly discolored from a run-in with a bowl of olive-juice, and the trackpad button sometimes sticks, causing constant mouse-clicks.
That said, it's perfectly usable for my purposes at the moment. I'm starting to consider possibilities for a replacement, though. These are my conclusions:
My current machine is 5lbs with a 13.3" screen. (My work laptop is 4.5lbs, with 13.3" screen). Both of these are totally acceptable, though I could probably get a larger screen *or* lighter machine now. Battery life is important.
I've not outgrown the 120GB disk -- this means I could spring for a 128GB solid-state disk and see what all the fuss is about.
I'm looking only at ThinkPads right now. Those keyboards are just too nice, and they have generally solid Linux support.
The W series seems too heavy; T series looks just about right. I like the Core 2 Duo a bit better than the i5/i7 because of the lower power consumption (25W vs 35W).
The ThinkPad T400s seems just about right, but it looks like it's discontinued (http://shop.lenovo.com/us/notebooks/thinkpad/t-series/t400s). Other likely candidates are the T410 (on the heavy side) and T410s. The X201 is also a possibility, but it has a small screen (12.1").