I just played around with the most recent myfaces 1.1.3-snapshot and facelets 1.1.2 and it seems to be required to put a DOCTYPE element in the faces-config.xml of seam
<!DOCTYPE faces-config PUBLIC "-//Sun Microsystems, Inc.//DTD JavaServer Faces Config 1.1//EN" "http://java.sun.com/dtd/web-facesconfig_1_1.dtd">
I fixed that in CVS