Often times, IE6 users don't know that they are missing anything because they are on IE6 and have nothing to compare to. A message of "this feature is not supported on your browser" or, preferably, "this feature requires a modern browser with support for HTML X and CSS Y" would go a long way to communicating that, in a factual, non-confrontational way. I think the latter message is better because it gives those who would use the website a factual rationale to take to their IT departments as to why an upgrade would be good, rather than letting the IT department dismiss the website creators as IE6-hating.