I am certainly opposed to excluding browsers from a public facing site. There is nothing more irritating than going to a website and discovering they ONLY support IE because some dev somewhere couldn't make things "work".
As many of the other authors above have noted there is a considerable number of users out there who use a company imposed desktop build or install of IE6. Your best bet is always to identify and communicate with your users, not impose your draconian concepts upon them.
Ryan Farley had an entry about this recently which describes what I think is the best first step to transitioning over users to a different browser. It encourages people to upgrade and explains why things may not render correctly in one graphic. Many years ago, BinaryBonsai.com was the first blog I encountered which had a badge appear suggesting FireFox and I totally downloaded it just not to be bothered with an additional graphic.
There really is nothing like nerd peer-pressure.