Jerome,
You can use "min_valence" and "max_valence" parameters on playlist/static just like the other attributes. We haven't documented it because valence is considered a beta attribute, but it can be quite revealing in some applications.
Here's a happy-ish playlist based on my favorite Mellow Gold artist. (Just add your API key.)
http://developer.echonest.com/api/v4/playlist/static?results=10&api_key=API_KEY&type=artist-radio&bucket=audio_summary&limit=true&artist=Billy%20Joel&min_valence=0.5&min_energy=0.5&bucket=id:spotify)
Enjoy, and Be Happy!