Neither SQL Server 2000, SQL Server 2005 or SQL Server 2008 is able to list existing mount points or mount point info (like total disk space and free space) through xp_fixeddrives or xp_available media or other extended stored procedures.
Neither does the Management Studio mark mount points as such, f.x. during the "New Database" dialog when you want to control where database files are stored this information would be nice - plus of course, a way to easily check that the wanted / needed space actually are available...
Currently the only way of identiftying that a given folder is a mount point through Management Studio is when the "System Volume Information" folder is shown in a subfolder.
Double mounting a drive through both a drive letter and a mount point is not an option on a cluster with many instances or luns attached + plus it may confuse backup systems, antivira software etc when the same drive is accessible through different paths.
Further there is a small problem with xp_fixeddrives and xp_availablemedia since they list cluster disk resources outside the scope of the current sql server instance.