We used the tabletop DFRPG system, actually. We've run a Dresden LARP at GenCon for the past two years now-- last year was 'City of a Thousand Cross-Roads,' while this year was 'City of Abandoned Hope.' Our games were 3 nights in a row, around 50 players each, and the system handled beautifully. Both games have been, hands-down, some of the best games I've ever had the pleasure of running-- a lot of that has to do with how positive and collaborative the FATE system is, and how it encourages good player interaction.
Before I ran DF, I was involved with Darkness Remembered White Wolf LARPs for a number of years, and before that, I spent some time as a player in Dark Duality LARPs-- if you spent any time with either group, we may have crossed paths.