This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
SingleThreadModel was an attempt at making it easier to to write thread safe servlets. The idea didn't really work out and the interface is now deprecated. It is worth understanding but only for maintaining older applications that employ it.
If you're interested in learning more about it, there is a link to the servlet spec in my signature. A quick search on that document for SingleThreadModel will bring you right to the relevant section.