A first (small) step has been made on the way towards running lib-gwt-svg apps into IE9. I have done some tests with the latest code available (called “platform preview 7”). The good news first: a few of the tabs in lib-gwt-svg-samples work, with the present unmodified code (version 0.5.0): shapes, events, features. This is encouraging because it means you can create SVG shapes and receive events. The bad news now: all the other apps are not working (yet).
How do I interpret these results ? It seems IE9 is a two headed beast, and these two heads do not talk to each other at all. Either you choose the new IE9 mode, by putting the HTML5 doctype header in your document; or you choose the compatibility mode.
In IE9 mode, you must do everything by the standards (CSS layout, HTML5, W3C DOM APIs). If you deviate a tiny bit or try to use the old IE way, it won’t work, contrary to other browsers which know how to handle legacy and degrade more gracefully.
In compatibility mode, you basically have that outdated and full of quirks and bugs browser, which knows nothing of the new features but has good compatibility with legacy MS apps.
So you cannot have your cake and eat it too. Currently this is not very good for the combination of GWT and lib-gwt-svg. GWT has not evolved yet, its widget library is designed for the old version of the browser, so GWT apps probably will work better in compatibility mode. However lib-gwt-svg is clearly designed to work in IE9 mode, because it requires SVG, only available in this mode. This is why (I think) so few apps currently work: as soon as you start doing GWT UI in IE9 mode, it breaks. Probably the GWT team will have to address IE9 in IE9 mode as a completely new user agent, which is a lot of work. There are chances most apps will run unmodified once this done. To encourage google engineers, you can vote for issue 5125 to show your interest in the topic (click on the star next to the issue number).