I think it was a quote I remember from a many years ago issue of the Grantsmanship Center News that said: the only thing wrong with tainted money is there tain't enough of it.

I guess that's the takeaway one can assume from a Feb. 20, 2008 Nonprofit Times article entitled Nonprofits Cash In On Questionable Contributions:

Well, here’s one way for charities to cash in on the presidential race. Nonprofits have been the recipients of cash contributions originally intended for campaigns...until the donors found themselves in hot water and committees looked to distance themselves by giving up the donations.

The campaign of Democratic presidential candidate Sen. Barack Obama said it would donate more than $40,000 to nonprofits, contributions received from a former friend and fundraiser who pleaded not guilty to fraud charges.

A spokesman for the Obama campaign told the Chicago Tribune that more than $44,000 in donations related to Chicago businessman Tony Resko, referred to as a slumlord by Obama’s rival for the nomination Sen. Hillary Rodham Clinton, has been divested.
It seems to me that a nonprofit board should have the same conversation Obama's campaign had: when is tainted money "keepable"?