Every year departments receive an appropriation of funds. They must live within that budget. If in fact the money isn't spent, there is a provision to allow, at least for operating and maintenance, a 5% carry forward. That's to prevent that ramp-up of spending at the end of the fiscal year, but anything over that, those funds do what we call lapse, so they go back into the consolidated fund.
Last year, for example, in 2005-06, $5 billion in fact lapsed, and that money would be available for other uses.