The Open Source Observatory and Repository for European public administrations (OSOR) published a case study on the french police (Gendarmerie) desktop migration to Ubuntu. Under the title "Towards the freedom of the operating system: the French Gendarmerie goes to Ubuntu ", they publish a summary of the project as gathered from a few sources within Gendarmerie and Canonical (hey, that was me!).
From the political and organizational aspects to the techical issues and the lessons learned, I believe that this case study is a must read for anyone considering or planning such a migration. Here are a few parts which I particularly enjoyed:
"If the French National Assembly gets its way, the open-source Linux operating system will take over the governments of Europe, seizing on a weak economy to displace Windows."