I'm in a hospital with a pretty awesome infrastructure environment ... Lots of HA, clustered servers, and we keep moving to newer servers as they come out -- AD on 2008R2, going to 2012, Exchange 2010, looking at 2013, SCOM 2007, going to 2012.
We have ~4,000 desktops & 500 laptops mostly running XP. Our hands are tied by the 3rd-party vendors that are providing most of the hospital's apps -- we have some apps running on SQL Server 2000 still (and some on 2012). After extensive testing, we've uncovered a few apps that either don't run on Win7/IE8 or (more importantly) aren't supported by the vendor on Win7 or IE8. Clinical apps are life & death critical, so vendor support is paramount.
That said, we also have a large Citrix deployment, so we can move ahead with Windows 7 & IE8, back-filling with Citrix-based applications on XP or IE7....