http://support.microsoft.com/kb/978506
http://dev-answers.blogspot.com/2009/04/ie8-compatibility-issues-and-working.html

Basically, it seems Microsoft has decided that they're going to change the way they view web pages – to make them stick to standards they weren't obeying before. At the same time they've stopped interpreting the old non-standard methods they created for ie6 and ie7 and implemented a stricter conformation to the standards than the other major browsers.

The problem is that GWT compiles JavaScript explicitly for IE browsers to make them use the Microsoft non-standard method of doing things, so the JavaScript that gets run when you visit GB2 in IE8 is explicitly compiled for IE6/7/8 prior to this most recent change.

Solutions:
(1) *Near-term/Workaround* From what I've read Microsoft has added a new "Compatibility View" button to the browser, which you can click to make the page render in the old way on a site-by-site basis.

(2) *Medium-term* Apparently, we can add a meta tag to the page:

<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />

(3) *Long-term* Almost certainly Google will roll out a fix to this to make GWT compile correctly for the new IE8 compatibility mode. Once they do, we can upgrade to that version and remove the meta tag from the fix above.