I thought it was because debt service was off-budget. (I haven’t been able to find a line item in the budget for that.)
No, the chief reason is because the general fund is borrowing the Social Security surpluses (raiding the so-called 'lockbox') and surpluses from other trust funds. This borrowing is correctly accounted for in the gross federal debt, the debt that is now almost $9 trillion. However, it is not accounted for in the "unified deficit". The following graph shows various measures of the deficit.
The actual numbers and sources are at http://home.att.net/~rdavis2/def08.html. The purple line is the "unified deficit" and is equal to all revenues minus all outlays. These revenues and outlays include the general fund and the trust funds. The blue line is the change in the debt held by the public and is very nearly equal to the unified deficit. The green line is equal to the public deficit minus the Social Security surpluses that are borrowed by the general fund. Finally, the red line is the change in the gross federal debt which is equal to the public deficit minus the surpluses that are borrowed from all of the trust funds. As stated, this is the debt that is now almost $9 trillion.
As can be seen in the graph, the unified deficit did become a surplus in 1998 through 2001. However, the "gross deficit" never did become positive though it came very close in 2000. Since the "gross deficit" is the change in the gross federal debt, this debt continued to increase every year.
The following table shows the money that has been borrowed from the trust funds. As can be seen, $309 billion was borrowed in 2006 and the total holding at the end of 2008 is projected to reach $4.2 trillion. This money is all "off-budget" in the sense that it doesn't show up in the unified deficit figures that are reported every year but do show up in the gross federal debt.
DEBT HELD BY GOVERNMENT ACCOUNTS (billions of dollars)
Investment or disinvestment Holdings
----------------------------- end of
2006 % of 2007 2008 2008 % of
Description actual total est. est. est. total
---------------------------------- ------ ----- ------ ------ ------- -----
Old-age and survivors trust fund.. 177.0 57.2 180.2 203.6 2176.9 51.5
Civil service retirement & disabil 29.2 9.4 9.3 30.1 729.4 17.2
Hospital insurance trust fund..... 24.9 8.1 11.9 3.3 317.4 7.5
Military retirement trust fund.... 4.5 1.5 27.1 7.6 216.5 5.1
Disability insurance trust fund... 8.9 2.9 4.2 5.7 212.0 5.0
Medicare-eligible retiree health.. 19.9 6.4 23.5 24.1 120.3 2.8
Unemployment trust fund........... 11.4 3.7 12.8 12.0 91.0 2.2
Federal Deposit Insurance Corp.... 1.1 0.3 1.8 2.9 53.9 1.3
Employees life ins & health benfts 4.1 1.3 3.0 2.4 51.4 1.2
Supplementary medical insurance... 15.9 5.1 8.8 6.3 48.2 1.1
Postal Service retiree health fund 0.0 0.0 31.4 6.9 38.2 0.9
Housing and Urban Development..... -0.2 -0.1 -0.3 0.6 30.7 0.7
Nuclear waste disposal fund....... 1.0 0.3 0.1 0.9 19.7 0.5
Highway and Airport trust funds... 0.6 0.2 1.5 -1.8 18.6 0.4
Exchange stabilization fund....... 0.5 0.2 0.3 0.4 16.4 0.4
Foreign service retirement & disab 0.5 0.2 -0.5 0.1 13.5 0.3
Other government accounts......... 10.1 3.2 -12.8 0.6 75.9 1.8
---------------------------------- ------- ----- ------ ------ ------- -----
Total investment in Federal debt.. 309.3 100.0 302.1 305.6 4230.1 100.0
Source: Budget of the United States Government, FY 2008,
Analytical Perspectives, page 230, table 16-4