"Uncaught TypeError: Cannot read property 'style' of undefined"
Which, in the cases I saw, was caused by Ext.util.CSS.getRule returning undefined. A specific example that doesn't render/work at all is examples/form/dynamic.html. Verified this in Chrome 16 and Firefox 9.0.1 on OS X Lion.
Probably unrelated, but I also noticed that the CSS files in the 4.1 beta release are not minified/compressed at all and are about double the filesize.. is that just because this is a beta release, or will they remain like that in 4.1 Final?