The graph above comes from Brad DeLong in his post about the Treasury Department's recent press release on its plans to issue government debt (Treasury bills, notes, and bonds) for the coming quarter.
The Treasury press release concludes:
Based on current projections, Treasury expects to reach the debt ceiling in the last quarter of calendar year 2009. Given the uncertainty surrounding potential borrowing needs, Treasury will continue to keep Congress and financial market participants apprised of developments as the debt outstanding approaches the statutory limit.
Such borrowing is needed for the stimulus, but it is clearly not sustainable forever. The government can borrow at phenomenally low interest rates right now, but most of its borrowings are short and medium debt, which will need to be rolled over in the not too distant future, when interests rates may well be higher. Also, there is a growing demand for TIPS (Treasury Inflation Protected Securities) and the nominal debt service on those securities will automatically rise if and when inflation returns.