I had a problem with Geithner's tax issues... see here for a comment I made at the time.  I thought that Obama should have pulled his nomination, but do not consider his tax issues to be a criminal matter. 

In case you are not aware of the facts surrounding Geithner's tax problem, here's what happened.  Initially, he paid his income tax on the income in question, but failed to pay required self-employment tax which covers the half of Social Security and Medicare which would normally be paid by an employer.  I'm fairly familiar with self-employment tax, having gone thru an IRS audit for basically the same thing as Geithner.  It's easy to believe that he honestly did not know that he owed the tax.  At any rate, the IRS had gone after him for the self-employment tax for 2003 and 2004, and he had paid it in 2006. 

His ethical failure was that once he knew he owed the self employment tax in certain years, he failed to amend his tax returns for 2001 and 2002 where he had similar income, likely because the statute of limitations had expired and the IRS could not audit his returns for those years.  I do not believe it was an "honest mistake", it was a legal calculation.  A definite stain on his record, but it has been well covered, he has taken his hits for his ethical failure, he's paid what he owed, and for me the matter is closed.