When I am using a custom HttpModule/HttpHandler in IIS 7 under Vista, the handler does not fire. The fix is to change the application in IIS to the "Classic .NET AppPool". You can also use the DefaulAppPool by changing the web.config.
Also, IIS7 on Vista Client doesn't currently supports ASP.NET substitution caching while running in integrated mode. If you change it to classic mode it does.
