Hm. I had thought about a mobile server for a while, when I lean more toward the "many devices" approach to mobile computing. And, note, for me, this means "something with an internal battery", as it's not really mobile, to me, if it needs wall power or car adapter power in order to function. I always thought of it as more of an "in-backpack server" than "a pocketable server" though. Or perhaps PAN server. But it would be a file server, an network gateway, and maybe a few other things (VNC to it for running certain apps, for example).

I think it would be best to be about the size of a half-height 3.5" drive, an express card slot for peripherals, a few USB plugs (for keyboard, mouse, ethernet, USB optical drives, USB hard drives, etc.), mini- or micro-DVI plug (with DVI-A support), some form of storage expansion (I'd say a PCMCIA slot, because it can handle everything from SD card readers, to CF card adapters, to even a secondary express card adapter, but PCMCIA is going out of style ... so I dunno; I'd probably be happy with an SDHC slot), internal space for a 1.8" drive, a wifi module that can act as an access point, and maybe a bluetooth module (with DUN and FTP at least, but maybe more profiles as well). AND LOTS OF INTERNAL BATTERY LIFE ... I wouldn't mind if half of the internals were the battery :-)

I might go with ubuntu for it, but I think even more than ubuntu I'd probably go for a rather minimal debian install (for example, maybe not even installing X on it, since I would only expect to use the DVI port for the most basic and/or dire console based stuff ... and that assumes it's a PC-like architecture, if not, then an rs-232 console is good enough). The basic shell stuff, package management stuff, web server with webmin, NFS, Samba, NAT, and a few more things are probably enough for the base install. Just make sure it's easy to find repositories for installing things like drivers for your express cards and USB dongles, and other peripherals.

For CPU architecture? If you want it to be compatible with the most server software that you wouldn't have to do a ton of your own building for (esp. for for things like binary-only drivers that might come for some of the express cards), then I'd suggest SOME form of x86 processor. But, that doesn't necessarily give you great mobility options.

And, like I said, for me one of the tasks I'd put on this is "network gateway". One thing I'd love is to be able to put express cards in it from the various wireless carriers, and have the drivers give voice, sms/mms, and packet access. Then run a SIP server on it for accessing its voice capabilities, and a jabber server on it for accessing its SMS/MMS capabilities. Then, obviously, use the wifi access point functionality to route the packet data capabilities of the express card.

Now you've got a PAN server.