Mars is about 35 million miles from the earth -- and was closer on Aug 27, 2003 than at any time in the last several thousand years. Over two years, the distance varies from about 39 million to about 240 million miles. If you have a computer, you can download or purchase a program which will tell you down to the nearest few hundred kilometers for any time in last few thousand years (or into the future...). Such programs are mostly based on algorithms developed by the Naval Almanac (which is now distributed on a CD-ROM). (The Navy is into this historically as star positions were used for centuries along with a clock to find latitude and longitude at sea).

Sources:
USNO

USNO web ephemeris (1 AU = 93 million miles)

Right now, Mars is about 34.6 million miles away from Earth. I know this because of all the hoopla surrounding Mars being as close as it has been to the Earth in thousands of years. If you look on the web you should be able to find just about anything you'd want to know about Mars since it is getting a lot of attention right now, which will continue for several months since two rovers are on their way to Mars.