HEXUS.net : Review : Intel’s BTX Form Factor - An Analysis and Opinion is a worthwhile article that goes beyond some of the “first look” coverage we’ve seen to date.
If you’re interested in BTX form factor and how it may affect you, this is a good article to check out. I can boil it down for you into two main ideas: First, don’t expect ATX to die a rapid death. Second, Intel isn’t aiming for a BTX revolution–instead they seem willing to alter the platform to the requirements of the market, which is a bit of a change for Intel.
If you want specifics of what exactly BTX is about, your best source is formfactors.org.