Primarily colonized by those doing the Great Divide and some Iditarod racers.
mtbr.com of course
Didn't see crazyguyonbike.com for touring. Definitely a sensation.
Local bike groups are also one of the best sources of infromation since gear availability, road and trail conditions, weather and what not are highly regionalized.
When I'm touring I'll often find bicycle groups mailing lists for the areas or trails I'm planning on traveling though or on and ask them questions. They are invalueable and often as happy to provide info as I am to be able to ask it. Localied knowlege rules.
There are also a bunch of great mailing lists on various subjects: touring, bike company mailing lists, you name it. can't begin to give an overview but there is a whole lot of conversation out there. An order of magnitude more then the great niche that is this ultralight forum.
Oh... also, flickr.com. Not sure their forums are a good place for discussion but there are hundreds of bike related groups and they're very active, for photo sharing at least. Some discussion goes on in the comments on photos as well.