I have used various mini-notebooks and ultra-portables for image storage....well to be precise, for image storage, on-site reviewing of image details, and evening-time editing / organising of images to lessen the amount of time having to do that at home.
Currently I own a Compaq EVO N200, which has a 10" TFT screen (1024 * 786), 20 GB harddisk, 196 Mb of memory, and a P3-700 w/512kb of cache. Quiet a blistering little notebook, weighing in at a measle 1.2 Kilo's.
Batterylife is average, but I purchased a converter to plug it into the car's 12v socket.
Before this one, I owned various Toshiba Libretto's, which are really only suited for image storage and catalogueing them, but not for editing (like I still ocasionally do on my EVO N200) . These little buggers are about as big as a videotape, and weigh in at 750-900 grams, depending on the model.
Mostly available on the second hand market, costing very little.
Models of interest are the Libretto 50 and up. (pentium class). The fastest one of the "old" s tyle librettos is the Libretto 110 CDT (233 Intel Pentium MMx) .
I transfer images through PCMCIA adapters for optimum speed..the media I read from are the limit, not the connection used.
Also, a funky feature, is using your little mini-notebook for remote shooting. Purchasing a 5 yard Firewirecable, and having your S2 shoot away at a distance, in the field, for all kinds of purposes and wicked experimental styles is wonderfull. (small animal photography etc).