Since September 30, the federal government has remained open under legislative actions known as "Continuing Resolutions".

"The federal government operates on a fiscal year cycle, which starts on October 1 and continues to midnight September 30. By the end of the fiscal year, if Congress fails to pass regular appropriations or continuing resolutions or if the President fails to sign or vetoes any type of appropriations legislation, nonessential activities of the federal government may be forced to cease for lack of budget authority." -- Thomas Legislative Information System

It Has Happened Before
This would not be the first government shutdown caused by a budget impasse.

1981 - November: President Reagan vetoed a CR. 400,000 Federal employees went home at lunch and told not to come back. A few hours later, President Reagan signed a new version of the CR and the workers were back the next morning.

1984 - October: No budget and 500,000 workers are sent home. An emergency spend bill has them all back at work the next day.

1986 - October: 500,000 workers are out for a half day. President Reagan signs a final appropriations bill and the workers are ordered back for the next shift.

1990 - October: The Government shuts down during the entire three-day Columbus Day weekend. Most workers were off anyway and an emergency spending bill signed by President Bush has them back at work Tuesday morning.

1995 - December: In the most serious shutdown in history, different functions of government are idled for varying lengths of time until April of 1996.