It looks like they are borrowing from the US Treasury general fund to pay for coupons, then in 2009 they will take the money from the auction fund, and put it into the general fund.
From; >> »
www.dtv.gov/DTVAct.pdf(i) ESTABLISHMENT OF FUND.--There is established in the
Treasury of the United States a fund to be known as the Digital
Television Transition and Public Safety Fund.
(ii) PROCEEDS FOR FUNDS.--Notwithstanding subparagraph
(A), the proceeds (including deposits and upfront payments from
successful bidders) from the use of a competitive bidding system
under this subsection with respect to recovered analog spectrum shall
be deposited in the Digital Television Transition and Public Safety
Fund.
(iii) TRANSFER OF AMOUNT TO TREASURY.--On September
30, 2009, the Secretary shall transfer $7,363,000,000 from the Digital
Television Transition and Public Safety Fund to the general fund of the
Treasury.
(1) implement and administer a program through which households
in the United States may obtain coupons that can be applied toward
the purchase of digital-to-analog converter boxes; and
(2) make payments of not to exceed $990,000,000, in the
aggregate, through fiscal year 2009 to carry out that program from
the Digital Television Transition and Public Safety Fund established
under section 309(j)(8)(E) of the Communications Act of 1934 (47
U.S.C. 309(j)(8)(E)).
(b) Credit.--The Assistant Secretary may borrow from the Treasury
beginning on October 1, 2006 such sums as may be necessary, but not
to exceed $1,500,000,000, to implement this section. The Assistant
Secretary shall reimburse the Treasury, without interest, as funds are
deposited into the Digital Television Transition and Public Safety Fund.