Ok, I have spent a while on this problem and this is what I have gathered:
If you make an AJAX call in IE7 and you have a window.onbeforeunload function spe
I figured that you just need to unset the window.onbeforeunload function before you do anything and then put it back when you're done.
I just ended up disabling the feature in IE.
did you try removing handler from "onbeforeunload" event before calling to window.open? This can help but I never tested it.