Thanks, FastBit looks quite interesting, but the site is so... academic. So much great research is lurking behind "unassuming" web presences. 'Someone should do something!'
http://www.freepatentsonline.com/6831575.html
Basically the patent covers the method of run length encoding. Each bitmap is stored as an array of words (32/64 bit ints), with each word being either a "fill word" telling it that the next x words are all 0/1 or a "literal word" containing an actual sequence of 0's and 1's.
Its a pretty sorry state of affairs when core, relatively simple data structures are patentable. Imagine if someone had patented the linked-list or binary search tree!
Interestingly, FastBit (which uses this IP) is released under the LGPL
[Edit: improved clarity]