Where can I find "hacker" information about the NXT?
You can start with the Hardware Developer Kit which includes "documentation and schematics for the NXT and related sensors", supplied by The LEGO group to help you "design and develop your own sensors and actuators [motors] that can interact with the NXT".

I've not really looked at the latest ones, but the original O'Reilly Unofficial Guide to LEGO Mindstorms was very good, and covered things like creating your own sensors for The Robotics Invention System - looking at the descriptions of the newer ones, they don't appear to require soldering irons or alternative programming environments :(.