I am attempting to accomplish single sign-on using two separate applications (WebForms, MVC) that reside on the same box. One of the sites is on a separate subdomain (xxx.abc.com/abc.com).
I followed the steps outlined in this article http://www.codeproject.com/KB/web-security/aspnetsinglesignon.aspx 
I've set the machine key in both applications 
 
<machineKey validationKey="21F090935F6E49C2C797F69BBAAD8402ABD2EE0B667A8B44EA7DD4374267A75D" decryptionKey="ABAA84D7EC4BB56D75D217CECFFB9628809BDB8BF91CFCD64568A145BE59719F" validation="SHA1" decryption="AES"/>  
Set the cookie name, cookie path, etc....
Config A (webforms) <authentication mode="Forms"> <forms name=".ABCLOGIN" loginUrl="Signin.aspx" defaultUrl="Home.aspx" protection="All" timeou ...

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