I'm using IIS and have heard that I may need to configure IIS to accept Post request can someone walk me through this. The app works just fine as a stand alone it's only when the tab gets displayed that the problem occurs and I get the above error.
I have a thorough understanding of the postbackURL and the tab url as that is not the problem.
Are you sure it is IIS problem? Looks to me like you just have incorrect FBML. You are not allowed to have any <body> tags in your tab's FBML.
I don't know if this helps at all but I spent a day banging my head on a similar issue. the Canvas App (IFRAME) works fine. The url apps.facebook.com/appname/tab/ generates a page on its own but the tab didn't work despite the canvas URL being right apps.facebook.com/appname and the the tab being /tab/
In the end I traced the problem to a forgery protection setting within my app that checked all post requests coming in. So even though Posts were accepted the application tested them for forgery protection, which they obviously failed coming from Facebook.
I skipped this for the tab only and everything worked fine. Now I don't know you're environment but there might be something there that helps you. Hope so. Good luck.