Since O’Reilly took web 2.0 and trademarked it, a lot of people have been wondering what it means. It didn’t seem like a big deal until CMP started sending cease-and-desist letters to non-profit organizations doing web conferences.
My legal 2 cents (worth about that much) are just get the next version. If you feel Web 2.1 is too weak of an increment, go for bigger numbers. Anything short of 3.0 will do probably do well.