It's looking likely that Democrats and Republicans will not be able to find a compromise on a six month budget deal by Friday. That means on Saturday, National Parks, museums like the Smithsonian, even most of the IRS shut down. (If you're expecting a tax refund, it could take a while.)
You can tell it's serious this time. I've overheard conversations in grocery stores, on the subway, in the House cafeterias about who is an "essential" employer. If you are, the paychecks keep coming. If not, you stay home from work. It's unclear whether furloughed employees would get back pay.
For the official word, here's the memo: http://www.opm.gov/furlough2011/