I've mentioned this before, and it was discussed in a recent military mom forum. Pretty regularly, military members get letters from the Internal Revenue Service (IRS) asking them to verify dates that they were in tax-exempt combat zones. Thankfully, they're pretty easy forms. You just have to fill them out and send them back.
Please, please, do this. Even if your entire unit gets the same letter, and your unit finance people tell you that they'll take care of it, return the form anyway. There is no reason not to, and you will be protecting yourself in case something were to (gasp!) go wrong with your unit fixing the problem. Ultimately, you are responsible for the notice that you received, and you just don't need that sort of hassle in your life.
My husband has received these letters a handful of times. I still don't entirely understand, and I think it is annoying, but I'm trying to come around to the fact that it is no big deal. Just send the IRS what they want, and hopefully they'll leave you alone.