Interesting problem. Thanks for reporting your work-around Matthew. I'm sure that will be helpful for others that run in to this IE problem.
I have tried to reproduce the problem with IE8 running in IE7 mode, but was unable. Ajax still worked with "Run ActiveX" controls disabled. One thing to note, as of IE7, Ajax support should be native to the browser (not requiring ActiveX to work). For that to be the case, though, the "Enable Native XMLHTTP support" IE security setting does need to be checked. You may want to also check that setting your computers to ensure everything is configured correctly.
Hope that helps.