I am partial to the IBM ThinkPads, particularly the T-22/T-23 lines. This line features full form factor (i.e., decent screen size and a keyboard size) although they are thin and light. Given all the features offered, the T-22/T-23 notebooks weigh a reasonable 6 pounds. They also feature an ultra bay that can be used to swap out a variety of media components, including ZIP drives, DVD-ROM drives, CD-RW drives, floppy drives, etc.
The ThinkPads are not cheap, but then again, there's no reason to purchase top of the line. You can purchase one with a 1 Gig MHz processor, 30 Gigs disk, 128 MB RAM, running Windows XP Pro for about $2400. You can purchase a refurbished ThinkPad with similar features for about $1500.
I think that IBM produces the finest notebook keyboard and TFT screens. Their production quality is unmatched.
By the way, I'm also a fan of Dell Inspirons, but they generally weigh a couple of pounds more. If your dad is using his laptop while traveling every ounce of additional weight takes it toll. Believe me, I've been there.