Currently (1.1.0.CR1), the seam-gen build.xml has:
<property name="eclipse.wtp" value="n"/> <!--input addproperty="eclipse.wtp.new" message="Is this an Eclipse WTP workspace [${eclipse.wtp}] " validargs="y,n," defaultvalue="${eclipse.wtp}"/-->
James feeling was that WTP was currently too flakey to really be able to support at this time.