GWT2SWF intend to provide software bridge between GWT and FLASH/FLEX. It means that using GWT2SWF you will be able to communicate between GWT and FLASH/FLEX. GWT2SWF provides SWFWidget and FlashPlayer version detection.
Working fine for me on current versions of all browsers (on Windows) and GWT 2.0. Minimal documentation makes it harder, but check out the related site at http://code.google.com/p/gwt2swf/ for more info. There's a great blog entry for more advanced uses at http://lgrammel.blogspot.com/2010/02/i-am-working-on-project-that-integrates.html