GWT 1.4 RC2 has just released and the final version should be closed to this one (read here: http://googlewebtoolkit.blogspot.com/2007/08/google-web-toolkit-14-release-candidate.html#links)
I think that this blocking issue http://jira.jboss.org/jira/browse/JBSEAM-1748 must be closed in the upcoming 2.0.0 CR1 version of Seam, to allow developers to play with the newest GWT version.
Well vote for the issue then