The IE9 beta has just been released, and it work with the latest versions of the Bing Maps SDK's. However, there's an issue with AJAX SDK v6.2 that result in rendering problems of VEShape-objects (points, lines and polygons). This issue also affects some samples of Interactive SDK. The workaround is to either switch IE9 to compatibility mode or better upgrade Bing Maps AJAX control to v6.3.
Just change the script reference from:
Note: keep in mind that this still doesn't mean it's officially supported.