The problem is when you use CSS for interface elements (such as dialog boxes, UI controls, etc), a broken IE6 implementation may mean those users can't use the site at all.

How relevant this is to your income, of course, depends on the site. For a blog, it may not matter at all, but there's a good reason why Facebook is notifying IE6 users of a degraded experience. They use a ton of CSS/Javascript for their UI, and I'm sure the cost of supporting IE6 is very measurable and very high. But they probably can't just tell IE6 users to go away entirely.