In keeping with the GWT philosophy of making it possible to write no-compromise AJAX in any modern browser, GWT 2.3 (in progress) will support IE9. We're adding a new user.agent property "ie9" and the notion of fallback properties, so any browser-specific bindings not yet implemented in ie9 will fall back to ie8 and show a compile-time warning that a fallback impl was used.
In the mean time, you can tell IE9 to behave as IE8 by inserting the following in the head section of your GWT host page:
<meta http-equiv="X-UA-Compatible" content="IE=8" />
Assuming that IE continues to maintain backward compatibility, you may want to insert the equivalent tag for IE9 once GWT 2.3 ships in order to make your app somewhat future-proof for future IE versions. For more information on document compatibility in Internet Explorer, see http://msdn.microsoft.com/en-us/library/cc288325(VS.85).aspx
The IE9 dev mode issue reported back in Dec is fixed in GWT 2.2.0: http://googlewebtoolkit.blogspot.com/2010/12/support-for-gwt-dev-mode-in-ie-9.html
You can follow the status of known IE9 issues here: