My organization is building a new version of our ticketing site and is looking for the best way to build an online waiting room when the number of users in our purchase path exceeds a certain limit. The best version of this queue would let new users in after existing users have either completed their purchase or have exceeded a timeout limit after entering the path.

I'm trying to get an idea of how this has been implemented by other organizations. Has anyone out there done something similar or have any experience with this? We have some ideas, but I'd like to get a sense of what solutions have been tried and what problems those solutions have run up against.

Just to be complete, this site is being built in Ruby on Rails, though I'd love to hear about how people have solved this regardless of platform.