Microsoft has since changed the rules with every version of Internet Explorer since (IE5.5, IE6, IE7), ignoring what they'd agreed would be the standards until IE8 (which is pending release). Finally, we have a Microsoft browser that plays by the rules and lets us move the ability to create stunning websites using Cascading Style Sheets (CSS) as they were intended, not just to pretty up the text.
“IE6 is the new Netscape 4. The hacks needed to support IE6 are increasingly viewed as excess freight. Like Netscape 4 in 2000, IE6 is perceived to be holding back the web.” Jeff Zeldman, standards guruIn the meantime, web designers have had to create hacks to create pages that would view properly on a variety of versions of IE, primarily IE6. I can't tell you the frustration of trying to move ahead into a design that doesn't use tables for layout only to find that IE adds extra padding or cuts off a graphic that has been floated to the right or left of text (the guillotine effect). Position Everything has a page devoted to the multitude of CSS bugs exhibited by IE.
Now, with the release of IE8, it is time to let IE6 die. There is a community effort to have IE6 finally removed.
However, there are a number of issues, many of which were listed in the article (unfortunately, no longer available) on the site related to the image on the left, Bring Down IE6.
Many people running older Hewlett Packard multifunction printers found this out when their computers updated to IE7. Microsoft had not only made IE part of the installer for all Windows programs (now if that isn't a foolish move in terms of security, I don't know what is) but HP then compounded it by linking their printing/scanning software to a particular version of a web browser. It probably wasn't' their fault alone. Microsoft probably rebuilt the engine for IE and never gave a thought to what the impact would be on the folks they'd leave behind when things changed.
So where does that leave us? It depends upon your situation. If you're catering to a corporate intranet (internal network) that uses IE6, then you'll need to fix the site accordingly. However, most of us can probably allow the site to degrade gracefully so that content is still available, but the experience is neither as rich nor pleasant for those using outdated browsers. There will be unexpected gaps where padding is added/removed and perhaps the graphics won't display as expected.
However, we've done this for Netscape 4, just like the Bring Down IE6 folks noted. Of course, Netscape wasn't the backbone of a cell phone access in a country like China either. Your decision is going to be flawed whatever it is. You'll just have to bit the bullet, either in extra costs associated with finding and fixing the bugs in IE6 or in a reduced experience for potential visitors.