:help CommandT

  ...
                                                   *:CommandTBuffer*
  |:CommandTBuffer|Brings up the Command-T buffer window.
  This works exactly like the standard file window,
  except that the selection is limited to files that
  you already have open in buffers.

and then later...

  MAPPINGS                             *command-t-mappings*

  By default Command-T comes with only two mappings:

    <Leader>t     bring up the Command-T file window
    <Leader>b     bring up the Command-T buffer window

Note that this is only available since 1.1b which was released in March.