See my other comments down there. In any case, not every thing everyone does should be open sourced. There are things you just have to hire smart people to do the jobs right and open source does not solve issues immediately.
That has miltitary secrets that you wouldn't want other govt to know and be able to utilize in their own development of fighter jets.
In the case of web code, there's nothing's new and innovative about the software that is unknown by existing developers. There is the potential for exploits but open source can help plug up those holes by allowing the community to assist in the process.
Sure, dumping the supposed "500 million" lines of code all at once would make it hard to review but they could release one compinent at a time over several months/years and vow to release all future code.
On a side note, wouldn't reviewing and repairing govt software be an excellent teaching tool for schools? How proud would a child be to have their code fix be accepted into production?
Great point, if something like this is done and students gets into learning and contributing to open source it is going to have a tremendous improvement on the quality of software.
They should have to make the case to keep it closed, not the other way around.