The BBC’s Responsive News team has a post out about how they differentiate between browsers, which they call cutting the mustard. They boil it down to “feature browsers” and “smart browsers”, or alternatively “HTML 4 browsers” and “HTML5 browsers”.
They walk through how they implement progressive enhancement, and load resources as they need them, using feature detection. Lanyard does something similar, as well. It’s an interesting approach, and I think could probably work for most clients, given you have an appropriate discussion about how different browsers support different features.