I work at Montefiore Medical Center in The Bronx, NY. Much of what I do is building intranet web apps. About a year ago they upgraded (yes, upgraded) to IE7. We have 18,000 employees, but let's say we have 10,000 computers. Upgrading all of those computers, verifying that all of the hundreds of apps provided by dozens of vendors used in scores of departments is a huge and expensive project. Healthcare in NYC is a business that earns you a margin of - if you're lucky - 1%. It is difficult to justify the cost
Note that I'm not a decision maker - in fact, I'm not even in the IT department. Please don't try to argue this with me, as I may be totally wrong about what their decision factors even were. Personally, my life would be much easier if they would upgrade, but I can understand why they have not.