Although they aren't APIs, www.data.gov provides official data sets, which can be mined. For now, I think this is the closest you're going to get to an official, centralized source of data.
Check out ProgrammableWeb's list of government-related APIs. Not all of them are the US federal government, so you might need to sift through it a bit. Also, they're not all provided directly by the government.
There's also an open source project that provides an API for thomas.loc.gov.