If you try to gain shared access to a remote login screen being used by the remote user they will be asked to give you permission do to so.

However, there is a way around this if you can log in to another account on the remote machine and also know the password for the current remote user's account. First, connect to a virtual display using your own account. Then log out. The standard lion login will show allowing you to login to the same account as the currently active remote user. At that point you can login to their account and get access to the user's remote screen without a permission request. Screen sharing does display a Status Menu on the right side of their menu indicating their screen is being shared.

Very handy when doing remote maintenance to other Lion Macs in my home. I don't have to walk to the remote Mac and give myself permission.

Originally I thought this was a security risk, but it isn't since you have to know the user's password to gain access in this fashion.