Since my previous laptop could not really be called "mobile," I recently purchased an IBM Thinkpad T43p. First, a comparison:

Dell Inspiron 9100 IBM Thinkpad T43p
Processor 2.8 Ghz Intel Pentium 4 520 (Prescott) 1.86 Ghz Intel Pentium M 750 (Dothan)
Memory 512 MB (2 Dimms) 512 MB PC2 4200 - 533 MHz (1 Dimm; 1 Free)
Hard Drive 60 GB 60 GB - 7200 RPM
Ethernet Broadcom 440x 10/100 Broadcom Tigon Gigabit
Wireless Dell Wireless 1350 WLAN Mini-PCI Card (Broadcom 4306 chipset) Intel Pro/Wireless 2915ABG
Video Card 64 MB ATI Radeon Mobilitiy 9700 64 MB ATI Radeon X300
Weight 8.92 lbs 5.14 lbs
Battery Life 1.5 hrs unknown (the IBM website says 4.9 hrs, but I have not done any benchmarks myself)

As you can see, the Thinkpad is better than the Inspiron in virtually every way (the Inspiron has a widescreen LCD panel, so it has WSVGA+ resolution (1680x1050) instead of SVGA+ (1400x1050). It also has some cool features (fingerprint reader, Active Protection System) that will not work in Linux. I might write more about configuring it for Linux sometime next month or in June.
I would be kicking myself for not getting a Thinkpad last summer (I was thinking about it) were it not for the fact that the Thinkpads on sale last August (if I wanted a machine for a comparable price to the Inspiron, I would have to have gone with a T41) were much less powerful. To an extent, this comparison is unfair, since the Inspiron 9100 is "last year's technology" (also, one could argue that there was a reason that Dell stopped selling it less than a year after it arrived on the market). I did look at the Dell Latitude D610, but decided against it, since it weighs more and has less battery life (also, my experience with my Inspiron has soured me somewhat on ever buying a Dell computer). This pair of reviews of the Latitude D610 and Thinkpad T43 played a large role in my decision-making.