An interesting Computerworld article reports on a survey by the Federal Open Source Alliance (Intel, Hewlett-Packard and Red Hat) whose responses indicate that more than half the US government's agencies are making use of open source software, and more are planning to do so. You can see more statistics in the report from NetworkWorld, though nobody says whether the sample was large enough to draw significant conclusions from.
Apparently the main drivers are the ability to customize open source packages and access to advanced security facilities, and 97% of respondents report success or partial success in their open source-based projects. This compares extremely well with projects based on commercial products.
Look for another survey next year, as the alliance plan to repeat the exercise and track open source's increasing penetration of government applications.