Okay, we all love to hate and fear the IRS, but hold on a moment. They have an ongoing effort to improve public relations, so take advantage of that.
Contact the Taxpayer Advocate Service and explain what happened. Tell them you cashed the check in good faith that it was due you, because they said it was. You not only shouldn't be charged the interest, you shouldn't have to pay it back.
Ask what can be done. I believe that when it's their error, they may have to let you keep the money. The advocate may be able to arrange that. Or you may need to file an appeal or a claim, but it should be a straightforward, simple one.
More info here:
http://www.irs.gov/advocate/article/0,,id=212313,00.html