I've not looked into it far enough, but does Aiport Extreme + USB Drive == Time Capsule?

The UK Apple store has the Extreme at £139 and a 1TB Time Capsule at £229. This gives you £90 to spend on an enclosure and disc. Jump to a 2TB Time Capsule and the difference is £240! Now that's quite a chunk of money to spend on a single 2TB disc. In fact, for me, that was halfway towards a bare ReadyNAS that also has a ton of additional features as well.

I'm sure it's just the paranoid pseudo-sysadmin in me but, but if I value 'any' data then sticking it on a single disc doesn't seem safe.

I can see your point Tom about giving them to relatives as an install and forget solution, but if you're a seasoned fiddler then it seems like there are possibly cheaper but probably better ways getting the same functionality.