I am trying to set up Single sign on for 2 websites that reside on the same domain

e.g.

http://mydomain (top level site that contains a forms-auth login page)

http://mydomain/admin (seperately developed website residing in a Virtual Application within the parent website)

Have read a few articles on Single Sign on e.g. http://www.codeproject.com/KB/aspnet/SingleSignon.aspx

And they seem to suggest it is just a case of having the same machinekey section in each web.config so that the cookie encryprion and decryption is the same for each application

I have set this up and I never get prompted for credentials in the sub-website (the virtual application)

I always get prompted in the parent site.

In addition to having the same machinekey I've also tried adding the same <authentication> and <authorisation> elements

Any idea what I could be missing?