I understand what you're saying, but remember that they split GM into two companies - the one that had an IPO today, and the other one that was riddled with debt. The US taxpayer assumed that company and it's debt. That wasn't $5 billion, that was something well north of $50B. They'll recoup some of that today, and in the future, but they wont' recoup all of it, ever.
Essentially, the US taxpayer is eating whatever is left, plus the $5B (is it a little more than that?) equity stake. I'd have to do the math on paper, but I'm pretty confident that the way they've done it is costing taxpayers much more than any exposure they had from pension guarantee programs.
I agree though, this should be - IMHO - investigation number one when the gavel falls on the new Congress.
The 5 billion was only the unions share of the stimulus money that was given to GM so that it could pay us back for some of the money that they owed us. That does not include the "investment" we have in GM. That will probably be another 10 or 20 billion loser, depending on how the stock deals work out.