As mentioned briefly in the "Any games that include Ferrari 512S or Porsche 917K? Or just dreaming? " thread, there has recently been a sim race that ran for 24 real life hours. Using the rFactor WSC 1970 mod at LeMans.

I believe 24 hour sim races have been tried before, but disconnections have always been a problem unless you are running during a LAN.

There is an excellent article here: http://www.simhq.com/_motorsports4/motorsports_134a.html, with pics and a link to a youtube video.