I'm in the process of committing the IE9 GWT agent support.
http://googlewebtoolkit.blogspot.com/2011/03/update-on-gwt-support-for-ie...,
http://code.google.com/webtoolkit/doc/latest/DevGuideIE9.html
This is going into the features/gwt-upgrade branch and will get merged into master with
the smartgwt 3.0 upgrade.
NOTE: this will affect size and compile time of the application as another browser
permutation is added.