About 7 years ago, I did a Linux desktop pilot with a large ad firm in Chicago. It would have reduced their costs, improved reliability, etc. However, the IT Director at the time did everything she could to work against me. She wouldn't enable IMAP on the server, which made switching off of Notes impossible. Further more, OpenOffice was 1.0 and the compatibility with Office docs was terrible, and it's not a whole helluva lot better today. Almost every other app had a viable open source replacement, but the Office suite was the Achilles heel.
If the open source community could get a full replacement for the Office suite, especially Outlook, it wouldn't be as hard to switch to Linux on the desktop. Maybe now that Oracle owns OpenOffice, we'll get better compatibility with Office files. However, compatibility isn't enough as "me too" products don't do very well in the market place. We need something that has enough "killer features" compelling enough to switch off of Office and Windows.