I'm in a bit of a bind regarding portable computing devices. My Powerbook is too big to lug around everywhere. I have a Sharp Zaurus, which is pretty much a full-featured Linux machine in a PDA package. It's still a bit big; I have to carry it around in a belt holster. I have a Sony Ericsson T637 cell phone. It's very small; fits in my pocket. It has alarms and a phone book, but is only about 75% of the way there as far as PDA functions. Part of the reason the cell phone isn't a sufficient PDA platform is software, but part is hardware: a tiny display and painful input method. given the hardware limitations, I don't blame Sony for not including more robust software.
Well, one part of the hardware problem appears to be solved in the near future. The linked New York Times article describes flexible display technology that Philips Polymer Vision will be bringing to the market within a couple years (they apparently will be showing a prototype device in a couple months). Basically, these (initially 5" diagonal) displays can be rolled up inside the body of a device, where a small portion could be exposed to view. You can then pull the full display out when you need it. Finally, a decent-sized display that fits in a (normal) packet.