Just received the same message from Shawn White saying it's "imperative" to return the call immediately! I'm so glad others have posted comments. I don't return calls until after I read this site!! Thank you everyone.
Official IRS Government site detailing this scam and providing links to report it:
http://www.irs.gov/uac/Newsroom/IRS-Warns-of-Pervasive-Telephone-Scam