Internet Explorer does not show contents of App_offline.htm

It happens because under default configuration, Internet Explorer first looks at the page size and if it's less than 512 bytes 404 status code friendly message is displayed instead of your App_offline.htm content.

So if you use the app_offline.htm feature, you should make sure you have at least 512 bytes of content within it to make sure that IE shows its content instead of 404 status code friendly message.

Configure "Show Friendly Http Errors" feature of IE6.

This can be configured in the Tools->Internet Options->Advanced tab within IE, and is on by default with IE6.