A clustered index physically stores the data in the sequence given in the index definition. The way this is implemented in SQL Server means you can only have 1 clustered index per table.

This is true in most other DBMSs, but not all of them. DB2 has supported multiple clustered indexes for some years (for *nix and Windows) and now also on the mainframe. You can define a large number (256?) cluster indexes on the same table. The data is stored only once, but is physically ordered by the sequence defined for each index.