From Kiplinger's just in time for the start of tax season:
The IRS gets many reports of cash transactions in excess of $10,000 involving banks, casinos, car dealers and other businesses, plus suspicious-activity reports from banks and disclosures of foreign accounts... Read more
These currency transaction reports are a valuable source of audit leads for sniffing out unreported income. The IRS agrees, and it will make greater use of these forms in its audit process. So if you make large cash purchases or deposits, be prepared for IRS scrutiny.