I think lazy might be a bit harsh. We are a large hospital and healthcare dollars are a precious resource. We are in the process of upgrading to Win7, but it will still take another year or two before we're done. I don't work in IT here, but I've heard estimates of about 50,000 desktops. Plus, we have a lot of homegrown software that needed to be modified prior to upgrading. That was a lot of work and a lot of money spent. We can't upgrade just because a new operating system comes out, even though that would make me happy - I'm the type that needs the latest and greatest all the time.
MS is still supporting XP, and yes it is good enough for now.