The offline functionality is cleverly dubbed offmeister and is built upon the
Google Gears library which provides a local server to cache and serve the application from your PC and synchronizes all changes when you are online again.
offmeister is available to all Premium subscribers, non paying members can try the functionality for 30 days. The ability to work on your mind maps whenever you want and whereever you are is (unlike the random and rather annoying limitation to 6 mind maps) probably a good selling point for the Premium membership which comes at about 40€ per year, mind mapping is all about capturing those creative juices flowing out of your brain after all and you never know when inspiration hits you.
Related post: MindMeister Launches MeisterAPI
Company blog: Blog
(This article originally was written for blognation Germany. Since blognation is gone I have reposted it here)