This is a great discussion. My customers are car dealers and while IE6 useage is declining, it is still 12.5% of my traffic. I have actually seen PC's running Win98 in these stores. Amazing!
We've segmented our catalog product into different interfaces for them to pick from. We're getting ready to launch a new one and we're close to making the decision that it will not support IE6. They can still choose one of the others that still do, and we will continue to support those but not enhance them.
So I can finally embrace sprites and transparencies and ... :)