In and Out will show up to your event for a few thou. Worth every penny

The "Minimum Charge" of $2,350 buys you 2 hours of time, but they don't do fries in the mobile unit. In 'N Out Burger Mobile Unit Agreement [pdf, in-n-out.com]
Learned about from http://positiveapeindex.blogspot.com/2006/07/2006-banks-gearhead-invitational_30.html