No one can really say, but the IRS does report on the number of returns that it examines.  For 2008, of the 138 million tax returns filed by individuals, the IRS examined 1,391,581.   This implies that you have about a 1% chance of being audited.  The most highly examined returns were those with more than a million dollars of income of which 5.6% were audited (down from 9.3% in 2007).

You can check out more of this information in the IRS Data Book, FY 2008, Publication 55b.