[citation][nom]fuser[/nom]Large public websites cannot afford to drop IE6 support. You can force your website visitors to upgrade or go away, but there's always the risk that users will pick the latter option.[/citation]
Two years ago, you would have been correct. If newer browsers cost the user money, you might be correct. But since Vista shipped with IE7 over 2 years ago and IE7, IE8, Firefox, Opera, Safari, Chrome, and other browsers are all available for free, it's a non issue today.
The only support you need for IE6 or earlier (and this applies to other browsers that are 2+ years old also) is a screen that tells the user whichever of these messages is appropriate.
1. This site works best with xxx, yyy, and zzz browser(s). Some features may not be available to you or may not work correctly with your current browser. We recommend you upgrade to one of the above browsers.
2. This site will not work correctly with your browser. This site works best with xxx, yyy, and zzz browser(s), please install one of these browsers and come back.
It's not any more of an issue than sites that require at least xxx version of Adobe Flash.