Comment by david.nouls: The part about "Exacerbating Factors" is not 100% correct.
Sure "filter:opacity(...)" or "filter:progid:DXImageTransform..." will use a lot of memory in IE6/7 but .. in IE7 there is no need to use them! There are bugs reported on this - soft permutations were promised to handle this. Also don't forget that if you run in quircksmode on IE8, that it also falls back on IE6/7 behavior.
For more information: http://code.google.com/p/google-web-toolkit/wiki/UnderstandingMemoryLeaks -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
