The issue is that this transaction is a debt-asset swap and comes with a distressed asset tax (DAT) of $7 billion. The DAT was codified in 1986 to prevent companies from buying money-losing companies just to avoid paying taxes. In GM's case, the debt-asset swap counts as corporate income, but GM can claim it's 2008 losses against that income, greatly reducing its tax bill.
If the tax isn't waived, GM will need to immediately return $7 billion of the money it was just given. It is talking to the Treasury Department, but so far it's been no dice. GM has been lobbying to have a waiver provision put in the economic stimulus bill currently being wrangled over in the Senate, yet there's also been no movement there, either. It's almost inconceivable that the government will demand GM pay the tax. It's equally hard to believe that this is even taking place.
[Source: Detroit News via Market Watch]