Unfortunately there is the real issue of moving over to opensource when you are firmly entrenched in a Windows culture. I work in support for desktops and servers for many thousands of NHS workers, and I can honestly say than you could probably tell them that you're going to upgrade them to the next version of windows, and install linux instead. They wouldn't know any different with KDE on. Seriously, most desktop users really don't care. There is a lot of truth though about the customized applications for windows desktop and server that would make any kind of migration a nightmare.
It could be done and would take a government mandate to force the issue. It would be a painful transition to OSS but I firmly believe it would be a better move than sticking with Microsoft in the long run. However, I've yet to see any realistic migration plan from an OSS community that would allow a large organization or company to migrate to OSS and it not cost an arm and a leg in support in the short term. Short term pain, long term gain. This isn't the mentality of world culture anymore. If it was then we wouldn't be facing the credit crunch. Something has to change and government policy is a good place to start.