There are 24 hour races all around the country. Granny Gear puts together one of the best known series and does a great job of it. Here is their 2005 schedule:
April 23 & 24 - 24 Hours of Temecula, CA
June 18 & 19 - 24 Hours of Big Bear, Hazelton, WV
Sept 17 & 18 - 24 Hours of The Ozarks (tentative)
Oct. 15 & 16 - 24 Hours of Moab, UT
If you want to sign up, the Granny Gear site has all the info you need.