Reports on Google's GWT forum indicate that OSX 10.5 Leopard introduces problems with GWT.
At this point, we don't know if this is caused by a problem in GWT, Java or OSX, but are continuing to research the issue. We will report back here as soon as we have more information.
In the mean time, we strongly recommend that OSX users not upgrade to 10.5 until this issue is resolved as GWT Designer will not work.