Or maybe he's un-endorsing it. See this mildly amusing exchange between Chuck Grassley and Tim Geithner midway through the latter's confirmation hearing today:*
Grassley: Did you use software to prepare your 2001 and 2002 tax returns?
Geithner: I did.
Grassley: Which brand?
Geithner: Before I answer, I would say that this was my fault, not the software's… I used TurboTax perparer.
Grassley: Did the software prompt you to pay self-employment taxes on your IMF income?
Geithner: Not to my recollection.
I guess one headline you want to avoid is, "The software did it," so at the very least, Geithner navigated that minefield...
As for the unpaid taxes issue itself, Geithner offered a semi-exculpatory piece of information today: In 2001, the year he left Treasury and joined the IMF, he not only received income from those institutions, but also the Council on Foreign Relations, where he worked briefly, and from various self-employment (presumably consulting) gigs. Geithner noted that he paid all the necessary self-employment taxes on the latter even though he whiffed on his IMF self-employment taxes, which suggests it was the result of confusion more than evasion. (Though I guess you could argue that someone truly devious might pay taxes on obvious self-employment income and evade it in more confusing cases because he could always plead "mix-up" down the road. But I think the burden of proof is on the person making that accusation.)
*My contemporaneous notes may not be perfect.