I had the same thing happen to me in late 2010. I contacted the big company's bored-sounding payroll department - after wading through the phone menu, and more than one "all of our operators are busy, please call back later <click>" - and told them I had this extra money. The guy in India told me that my petition would be investigated and that a ticket would be opened. I heard nothing for a couple of weeks. I followed up with payroll. They said that my petition investigation had determined that I did indeed get paid extra, and they'd be sending me a letter demanding the overpayment.
I received no letter, and a month later (January 2011) I got a W-2 with the paycheck included on it. I decided that I'd spent enough of my own time and effort on it, and if they wanted it back, the ball was in their court. I changed my bank account numbers to prevent them from auto-debiting my account, and spent the money as if it was mine.
I have not heard anything about it since then.
From what I was able to determine, once I'd made a good-faith effort to return it, I was in the clear.
And for what it's worth, it's not like you can just "return" it. Among other things:
- Withholdings were sent to the IRS in your name, and you have no way of getting that back
- Money was deposited in your 401(k) and you can't touch that until you're 59.5 years old.
- The amount they gave you will be included on your W-2, meaning you'll have to pay taxes on it.
- There may have been other withholdings for insurance or other employee-paid things; you have no way of getting that back.
I certainly wasn't just going to mail the company a check and hope for the best.