Dealing with people still using Internet Explorer 6 is a nightmare - so much extra coding and QA to get even basic webpages produced. I can't agree enough with Ben on his recent Mashable post.
Engineering productivity levels wouldn't double but it would probably go up by 35% - 45% to accommodate these 20% of Internet users who just won't upgrade. There are much faster ways of coding, doing quality checking, and assurance that teams can't use because of IE6.
The lowest common denominator of IE6 users is hurting progress in developing great, useful, fast and high-interaction AJAX pages. Maybe the seeming immortality of IE6 is one of the reasons we're still looking at pageviews and CPM for measurement in advertising. Everyone knows that with AJAX and FLASH, pageviews become meaningless since a page can be refreshed over and over, and a user can be shows dozens of ads without actually changing the URL of the page. So, a single pageview, but a complete user experience. But AJAX is very very slow and doesn't work a lot of the time on IE6, so as the browser that just won't go away, IE6 keeps development and user experience restricted.
Web developers - stop building for IE6, and throw up a huge note to tell users to upgrade. If content disappears, IE6 users won't have any options.
A call to all IE6 users - UPGRADE! IE8 is out now, nevermind the fact that IE7 is actually OK. Or use Mozilla Firefox or Google Chrome. They're both better than IE anyway.
You were great at one time, but please go away IE6, it's time.