OK, I've just reached a state of computer-system nirvana.
On the hardware side, I'm now using an Alphasmart Dana (bought on eBay for £100). The best way to describe this is combining the simplicity of an electronic typewriter with the convenience of a Psion organizer.
On software side, I use Google extensively (Gmail and Calendar). I use CompanionLink for Googleto synchronize my online calendar with the Psion desktop/Alphasmart calendar.
In terms of editing documents, I'm using OpenOffice 3.0 which looks wonderful. This has full support for mathematical formulae, and can export to MediaWiki or Latex format. It links smoothly with (open source) Zotero, which is a Firefox plugin for bibliography management.
Reluctantly, I'm using Microsoft Word .doc format again, just because everyone (including my Alphasmart) can use this.
This system seems to work very well, so now I'm going to use it!!