Google has just released GWT 1.7.1 to fix the issue I mentioned a few days ago regarding Snow Leopard compatibility.
In short, you can now run GWT on Snow Leopard using the Java command line argument -d32 without further modification. The GWT SDK no longer directs you to only use Java 5.
http://googlewebtoolkit.blogspot.com/2009/09/gwt-171-release-fixes-mac-os-x-snow.html