Free minecraft exploit

Video: http://www.youtube.com/watch?v=OVdk8NYQKIE (not mine, I was just searching for minecraft videos and came across it)

If you have a non-premium account (demo), and login to minecraft in the browser and the offline version at the same time (you have to login to the browser first), the downloaded minecraft client will ask if you want to use offline mode. This in turn gives you full access to minecraft because it doesn't know it's a demo account.

I haven't tried this myself, nor do I know how demo works, but I'm pretty sure this is a solid exploit, even after demo has ended.