Our QA discovered this while qual'ing some of my code. I have a popup page that include the button...
|<a4j:commandButton value="Cancel" styleClass="pushbutton" onclick="window.close();"/>|
QA found that if they launch this popup and then hit "cancel" for a total of 8 times in sequence on either IE 8 or IE 9 that the launching tab in IE will 'black list' the server within that tab. You can go to yahoo.com, facebook.com, etc successfully but *any* URL you use that points to the original server just results in the tab spinning. If you open another tab it will connect correctly. If you change your URL and use the IP of the server instead of the name it will work correctly (until you reproduce the issue again). The solution was to use h:commandButton.
Testing on Firefox and Chrome shows no problem.
I'm posting this mainly to help anyone who may run into this issue.