First off just wanted to say I think this is the next logical step from my original concept in the other thread (it's the same device with a slightly different application afterall), so I'm all for it. In terms of scale and battery, quite honestly I think there needs to be some more serious examinations of reality going on around here guys ;-) Yea, it'd be nice to be the same size as the tablet or smaller, but you really expect to achieve that AND have an internal Harddrive, USB hub, PCMCIA slot, etc...

The two hardware options I have in mind right now would consist of the following:
AMD Geode 500 Mhz processor... plenty of pump for a server, along with lots of power efficiency (for an x86 based processor).

2-3 fullsize USB ports... for, whatever. Extra card readers, devices, maybe a 3G modem if you're feeling lucky, whatever.

VGA output... For other uses mainly, not really beneficial for a "server", but would be very useful for other applications the device might take on

Internal USB Laptop Drive or CF card... both available in fairly large sizes for relatively cheap. (SDHC would probably come in the form of an extra USB card reader if desired)

1 hardwired ethernet port... for obvious reasons

802.11 wifi... again, obvious reasons here.

The whole battery thing... well, it introduces costs and physical size, that's all. Also, if you're talking about a mini HD, the battery is pretty pointless as you do NOT want to carry the thing around with the HD spinning... especially in a backpack.

Anyways, I'm persuing the hardware options right now, but it looks like it'd be somewhere around the $200-300 range, depending on whether it's got a HD or a CF card. Batteries/etc would add costs to the top of that, but again I haven't investigated battery options yet. Size wise the option with CF would be about 10cm x 15cm x 4cm-ish, the one with the HD is more like 15cm x 15cm x 10cm.

Oh, and for OS... I'd say something more like OpenWRT makes a lot more sense than Ubuntu or anything else, because it's designed to be very small/compact, quick to boot, and also very versatile in terms of packages and server type of software...

Just my two cents though...

-Rob