SPWeb.GetSubwebsForCurrentUser Method

As stated previously this does not work if a site has a child site with no permissions, but a site under that child site where the user does have permissions, it won't get the grandchild site. 

Try using 
SPSecurity.RunWithElevatedPrivileges 
(http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spsecurity.runwithelevatedprivileges.aspx)
and looping through the webs collection of the SPWeb object. Then run the DoesUserHavePermissions with the current user context to see what sites the user has rights too. 

This only works if the current SPWeb object is the RootWeb of the Site Collection.
Don;t expect any result when the current site is only a simple web.

Note that this will work only at the top or specified level if the user doesn't have permissions via inheritance in sharepoint.

If a user doesn't have permission at a top level site (parent) although they have permission in a subsite (child), the child will not get returned by this method.

Since the user doesn't have permission at the top level, it is hidden from the current user's context and therefore doesn't get queried by this method.