The HP 2133 is too large for a cargo pocket (the kind that comes to mind with reference to pants), but that small backpack you linked to looks roomy enough.
The HTC Advantage is a superb laptop replacement that gets you much longer battery life weighs much less. But I don't know of any way to get Linux on that with all the bits working (i.e. all the forms of wireless networking).
The iPAQ 211, given it's PDA form factor might not be up to extensive writing-- and you'd need an external keyboard. Also, I don't know of a Linux port for that device either.
That leaves the EeePC which would do all you want and can be had with Linux out of the box! It's got WiFi for Internet but there's no cellular WAN connection-- not sure if that's an issue for you.