Here is a direct quote from a little web site (Sponsored by the IRS?) called Community Dispatch.
You Can Help Shut Down Phishing Schemes
The good news is that you can help shut down these schemes and prevent others from being victimized. If you receive a suspicious e-mail that claims to come from the IRS, you can relay that e-mail to a new IRS mailbox, firstname.lastname@example.org. Follow instructions in the link below for sending the bogus e-mail to ensure that it retains critical elements found in the original e-mail. The IRS can use the information, URLs and links in the suspicious e-mails you send to trace the hosting Web site and alert authorities to help shut down the fraudulent sites. Unfortunately, due to the expected volume, the IRS will not be able to acknowledge receipt or respond to you.