Hide Content or Controls Based on Permissions
One of the little controls SharePoint possesses that gets missed often is the SPSecurityTrimmedControl (http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.webcontrols.spsecuritytrimmedcontrol.aspx) which is a wonderful little thing that actually hides whatever the content is based on the permissions you define.
As an example I can hide controls or content from anonymous users, and show them for logged in users using the following code:
<DIV>THIS TEXT IS HIDDEN UNLESS YOU ARE LOGGED IN</DIV>
This results in the DIV control and it’s contents only being displayed to users who have the BrowseDirectories Permission.
There are many ways this can be used from hiding navigation items, to the hiding the Sign In link effectively.
The permissions you can select and use for this control can be found here: http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spbasepermissions.aspx
Hope this helps,