Brazil does their Census with Windows Mobile PDAs
Over 5,000 cities and 82,000 Windows Mobile devices.
The application runs on the .NET compact framework and uses SQL Server CE.
The government saved time (6 months) and money and enjoyed significantly more accuracy while avoiding fraud. The Brazilian government is so happy witht he results, they expect expect to deploy 300,000 devices by 2010.
You can read the case study translation from portuguese here.