My verdict is, it’s not worth supporting any more. Those organizations that prevent their users from running any other browser probably also restrict the kinds of sites they’re allowed to visit any way, so I wouldn’t worry about it.

For years (i.e. since it came out, about 7 years ago, I think), Micrsoft’s Internet Explorer has been a source of frustration for web developers worldwide. Web developers have published LOTS of pages helping other developers circumvent the multitude of inadequacies and proprietary quirks it foisted on the unsuspecting web user. This site is a great example: http://www.positioniseverything.net/explorer.html
A google for “ie6 css problems” returns 715,000 results. My colleagues and I often discuss sending an invoice to Microsoft rather than forcing our web site customers to bear the cost of working around those IE6 problems, which can sometimes double the time required to create website. I wouldn’t be surprised if the international cost borne by businesses and organisations who, due to its lingering marketshare, still have to accommodate the fundamental brokenness of MS IE6, is in the tens or hundreds of billions of dollars. To their credit, Facebook have recently announced that their new site engine will no longer support IE6, as doing so would have meant too big a compromise on their ability to offer innovative functionality.

The fact the some businesses and organisations still use IE6 is inexcusable given the multitude of better options that are freely available (both free of cost, and free of the proprietary rubbish). Those who still cling to IE6 are simply demonstrating their lack of savvy. In most cases, their internal business systems (e.g. intranets) were built with dependencies on IE6-specific technologies (like Microsoft’s proprietary security nightmare: “ActiveX”). They are all shining examples of the foolishness of betting on the consistency of proprietary technologies, and the utter folly of equating “big corporation” with “long term stability”.