Error message when you perform an operation in a frame on a Web page in IE7: "Access denied"
To resolve this issue, set the document.domain property of each frame on the Web page to the same string together with the same case. For example, if you set the document.domain property of the first frame to "contoso.com," you must set the document.domain property of the second frame to "contoso.com." You cannot set the document.domain property of the second frame to "CONTOSO.com" or to "Contoso.com."
http://support.microsoft.com/default.aspx/kb/938302