I suppose that you are using RF 3.3.3 Final. Aren't you?
Had you been following instructions from http://community.jboss.org/wiki/RichFaces333andJSF20?
If this page(http://livedemo.exadel.com/richfaces-demo/richfaces/comboBox.jsf?c=comboBox) is working fine for you it probably means that there is a problem with incompatibility between RF 3.3.3 and MyFaces 2.0 . Are you facing this problem only with MyFaces?
It looks like is a conflict between the version of prototype.js that RF uses and the version that the Tomahawk lib is using internally. Unfortunately everyone has their own version.