The Terrorist Screening Center runs the list, accepting nominations of new names from a large range of government agencies.

Agencies use the watch list to check persons seeking visas, traveler entering or leaving the country, domestic airline passengers and persons stopped by state, local or federal law enforcement.

Most of the positive matches on the list came from police routinely checking persons, such as speeding motorists, according to the Government Accountability Office's report (.pdf). The number of individuals on the list is likely significantly less than 900,000, since the name count includes aliases.

Persons on the list are coded with varying suspected threat levels, so simply being on the watch list doesn't mean one will be arrested when say a person on the list pulled over for speeding. In fact, being on list isn't even enough to ensure that a person is denied a visa or entry into the country, according to the report.

The Transportation Security Agency exports two subsets of the list -- the no-fly and selectee lists -- which airlines use to decide who can get a boarding pass and who has to get hand-searched. The TSA continues to work on a much-delayed project now known as Secure Flight that would have the TSA perform the name checks in advance of a domestic flight, in an effort to reduce the number of falsely flagged travelers. Those affected over the last 7 years include many David Nelsons, a high-ranking nun, and federal employees with security clearances.

If the ACLU's math is correct, the list should pass the million name mark sometime in early July.