Assuming you are trying to sell large business/government software then IE6 will be a fact of life for many years to come. If you are dealing with the UK government, NHS, or local government in the UK then IE6 support is a given.
A lot of customers will not move from IE6 as they have other web apps that will only work on IE6, they are also not willing to install other browsers for control reasons.
However I have come across cases when the customers would install Silverlight on all of their machines. How common is this?
(I am not asking if you like IE6, or if you like Microsoft, Sorry we have to many money in the world as it is today.)
Some people think this is off-topic; however as a programmer I have to decide the best technology to use when asked to create a “web app”. Most users just think of “web app” = “no need to install” and “it runs in the browser window”, so this is an important questions when planning a product.