I'm using ExtJS 4.0.7 on Windows 7 Professional with Chrome 24 and IE9.
Like several users in the Bugs 4.x forum, I've discoverd that every http request is adding to the browsers memory. Garbage collection does not free all the memory so over long periods of time (4-8 hours or more), or frequent polling (20 sec/min) the application begins to starve.
Yesterday I migrated to 4.1 beta from 4.0.7 hoping to resolve the problem but with no success.
Several others have already posted example code exposing the issue so I won't add to the collection. ShadowZero3000's post from May 2011 exemplifies the problem.