Woo!  Just spent a few hours trying to get the Ajax toolkit and the Ajax AutoCompleteExtender working on our companies ISA server.  I placed the AjaxControlToolkit.dll into the Bin folder, but I was constantly getting "Done, with errors" in the IE browser.  Using the Safari browser and the Show Error Console, I found that the WebResource.axd file was not being pulled down, all because the path was not correct.  After many hours, I found that one of the ISA rules was not set correctly.  However, in my many hours of investigations, I found the following two web sites to be particularly helpful:
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