Thanks for all the answers guys - interesting stuff.
A further thought - does straddling only apply to cash games. I've played a few tourneys - and seen a fair few on TV too - and I've never come across it there, so I'm presuming it doesn't happen/isn't permitted. If that's the case does anyone know why not - because surely the same principles would still apply - i.e. aiming to loosen the game up and promote action.