According to the IRS in their News Release from last month:

IR-2011-39, April 7, 2011

WASHINGTON ?? Hiding income in offshore accounts, identity theft, return preparer fraud, and filing false or misleading tax forms top the annual list of ?dirty dozen? tax scams in 2011, the Internal Revenue Service announced today.

?The Dirty Dozen represents the worst of the worst tax scams,? IRS Commissioner Doug Shulman said. ?Don?t fall prey to these tax scams. They may look tempting, but these fraudulent deals end up hurting people who participate in them.?

The IRS works with the Justice Department to pursue and shut down perpetrators of these and other illegal scams. Promoters frequently end up facing heavy fines and imprisonment. Meanwhile, taxpayers who wittingly or unwittingly get involved with these schemes must repay all taxes due plus interest and penalties.

Here is the Dirty Dozen for 2011.