I just got this info from Sam Cleaver who is now the developer on Infinite Scroll (produced newest version that I use) regarding how to get it working on IE9:
Okay, IE9 requires a couple of fixes. First you need to change your next Selector to:
You'll also need to install a patch (a few code fixes for IE 9). Just download:
And overwrite the existing one in the infinite-scroll folder.
Since I do not have IE9 I am hoping you might still have it running on your test site. Any way you can check it out to see if this fixes it? I can make the changes but no way to test. Thanks