As much as we would like to turn a blind eye to the large number people using Internet Explorer and thus take advantage of CSS3 and HTML5 in all its glory, with certain projects, we really don’t have much of a choice.
If the traffic in a particular niche is IE-heavy, then you have to deal with that accordingly. If you go the Andy Clarke route then you may choose to use those new enhancements to the full, but allow a degraded experience in IE.
If you go the traditional corporate route, then you do everything you can to get IE to look and behave the same as the other browsers. That could mean ignoring a lot of new CSS3 and HTML5 stuff, or else it means filling in the gaps with scripts, hacks, and IE-only filters.