Now that you've read this, there are a number of other places on the web to further explore the Mindstorms. Most of these are not Linux specific, but most of them that have discussion forums probably have at least a few users who use Linux. Hopefully, I've stimulated you enough to explore some more. Enjoy!
The LEGO User Group NETwork site, http://www.lugnet.com/, has long been the center of the online Lego universe. It has great newsgroups with lots of people. In particular, the robotics forum has an avid group of very knowledgeable readers and posters. In addition, several of the options above have their own active newsgroups in the robotics/rcx/ hierarchy.
The official Mindstorms site at http://www.legomindstorms.com/ has lots of neat ideas, and even accepts NQC source for public distribution. However, don't expect gobs of info on Linux.
I've already mentioned these, but it can't hurt. These are the most comprehensive and detailed looks at the RCX hardware that are available to folks who aren't LEGO engineers. The "original" site is http://www.crynwr.com/lego-robotics/ and the second is http://graphics.stanford.edu/~kekoa/rcx
As you may know, the RIS was partially inspired by work done at MIT. The creator of the famous "Lego class" at MIT has a home page that includes not only links to the class (some seriously cool stuff in there!) but also to his Robot Builder's Guide, which has lots of great suggestions about actually building robots to execute all the great programs you've just learned how to write.