A large part of the problem is that while (nearly) everybody would benefit from IE6 fading away, there's a strong incentive for individual parties to still supp

A large part of the problem is that while (nearly) everybody would benefit from IE6 fading away, there's a strong incentive for individual parties to still support it (especially if they're the only one): blocking IE6 means losing a relatively large amount of customers. It's a prisoner's dilemma-esque situation. IMHO, the best solution I've heard is to offer a discount for customers who are not using IE6, and politely but clearly note that the cost is greater because IE6 is so old and buggy that supporting it requires a lot of extra work, "but switching is easy...[links].". Testing the user agent string is probably enough -- people who know how to change their user agent will know how to install a new browser, but probably can't because of e.g. IT department policies).