There seem to be two types of BDD.
The first is the original style that Dan North discusses and which caused the creation of the xBehave style frameworks. To me this style is primarily applicable for acceptance testing or specifications against domain objects.
The second style is what Dave Astels popularised and which, to me, is a new form of TDD which has some serious benefits. It focuses on behavior rather than testing and also small test classes, trying to get to the point where you basically have one line per specification (test) method. This style suits all levels of testing and can be done using any existing unit testing framework though newer frameworks (xSpec style) help focus one the behavior rather than testing.
There is also a BDD group which you might find useful:
http://groups.google.com/group/behaviordrivendevelopment/
