A multiseat system is a computer set up for multiple simultaneous GUI users, using multiple graphics cards, keyboards, and mice. They're perfect for Internet cafes, library catalogs, kiosk systems, home offices, and even gaming. I've been running a 3- or 4-user multiseat system at my house for almost five years.
PulseAudio, the new GDM, recent X.org releases, ConsoleKit -- Fedora is now a better base for Multiseat than ever before, but it still requires an enormous amount of configuration to get a multiseat system running well.
If you use multiseat systems, or are interested in using them, let's get together during the FUDCon F10 hackathon and plan out a roadmap for making Fedora the best multiseat platform ever.
(Basic -- and old -- multiseat configuration information is on my blog).