Rounded CSS Corners Without images

About a year or two ago I wrote a quick demo on doing rounded corners in just CSS without using images, mostly as an exercise to see what's involved. Since then my knowledge of CSS has doubled, maybe even tripled - so here I'm going to present 'cleaner' versions of those methods. I did a second example just a month back, and after presenting it on several forums I was linked to other methods of doing this, and decided to condense the best of all techniques down to a single easy to deploy version.