I've recently learned about Tomcat 7's feature to allow multiple versions of the same webapp deployed at the same time:
http://www.tomcatexpert.com/blog/2011/05/31/parallel-deployment-tomcat-7
http://www.javacodegeeks.com/2011/06/zero-downtime-deployment-and-rollback.html
Our sites regularly get 10-20,000 user sessions per day, and quite a lot of them are transactional/stateful type of webapps. Parallel deployment seems perfect for what we want, but I haven't really heard much about people's experiences using it on their servers.
If you use this feature of tomcat 7 in production, have you had any issues with it so far? Have you had to make any changes to your webapps to "play nice" with this Tomcat feature?