The idea and initial implementation looks good, but most of the complexity and implementation details are due to cross-browser issues. In particular, we left out functionality that we couldn't implement consistently across all browsers, and my recollection is that this was one of those cuts.
It is possible that has changed now, or that the set of browsers that don't support it is small enough we can add it with documentation explaining when it can't be used, but it definitely needs extensive testing across the full range of browsers GWT supports. http://gwt-code-reviews.appspot.com/1614809/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
