The Multiseat Display Manager is now out and (hopefully) helps you to set up a multiseat environment (much) easier than before.
I have always wondered when a software such as the MDM would come out, as the Unix and X architecture is already perfectly prepared for cases as this and even over Network it is already possible.
Now you need only one computer with several displays, keyboards and mice to let several users e.g. surf the internet at the same time.
I will post my experiences as soon as I get to trying it out. For the curious of you here is a link to installing instructions, packages for debian and ubuntu are provided, as well as the sources of couse.
Update: The packages is very large at 20K, but unfortunately at this point available only as i386 package...
Update2: I've quickly assembled a Ubuntu 8.04 package for AMD64 systems ;-).