Hate to be the pessimist here,
First, I have a hard time believing the IRS would be correcting a mistake from 7 years ago (yes 7 years, because while we're living in 2010, they're working on 2009 tax returns). Though it is possible.....just not probable
Second, when many people say they don't "owe" any taxes, what they mean is they overpaid on their withholding. That may make some feel good, but it means you loaned the government money tax free. So I wouldn't brag about that myself. My way of thinking is I want to owe them each year. Just under the amount that would incur a penalty. Yes, it bites, but it was my money until 4/15 and I could invest it, or pleasure myself.
Lastly, $800 happens to be the exact amount of the "Making Work Pay" tax credit for "joint/married" return. Did you take that credit?
But I do agree, it was a pleasant surprise, one you may have cheated yourself out of all these years, but its nice to have $800 in found money.