As long as your friend is not pulling you down with him than ignoring it is fine. I think it would be a problem if he had an effect on achieving your goals. I know somebody who likes to talk about what he wants to/should do, but he never does it. Yes, I know what you mean with the "sign me up" act that they like to pull on you. "This is going to be a such good idea, can you like work on 'insert stupid project that you know will fail from the get-go here'.
What I usually say when that happens to me is "Let me know how it's coming and I'll see if I can do anything." They never let me know how it's going ever again
, and plus it doesn't committ to anything. It basically puts the ball in thier court.