So, to cut a long story short;
- get the latest version of Eclipse 3 (I'm using 188.8.131.52)
- get the latest version of Flex 3 Builder beta Plugin (I'm using Flex 3 Builder public beta 2)
- install Eclipse as usual
- run the Flex installer
- use the shortcut under Programs -> Adobe to start Eclipse
Well, that's as far as I've got so far, I'm sure there'll be plenty more flexing going here in the future.