The GWT Plugin enables you to call methods on a Struts action class using GWT RPC. Documentation and How-To information is available here

With the release of GWT 1.4, this popular AJAX package from Google is much faster and its GWT RPC component is no longer tied intimately to servlets, and thus makes it much easier to use GWT directly with Spring. Here is an excellent blog entry that discusses using Spring and GWT together, complete with lots of example code and spring configurations. Here is another blog entry which describes 4-5 approaches for GWT and Spring integration.

It's fast!