IDL is supported on the linux cluster primarily for specific research groups in Astronomy and Cosmology. A few licenses are also provided by Physics and Astronomy for general use.

From Princeton Astronomy department: IDL hands out licenses based on hostname+username+display. If all three of those are equal you can start as many sessions as you want and only consume one license. The modern habit of letting ssh create X tunnels causes the display name to be different for each ssh connection, even if several connections come from a single host. There are several ways of making your DISPLAY name the same for all such connections. The simplest is to note which your first one is (echo $DISPLAY), and set DISPLAY on other connections to that. Or for a console-only IDL session, simply unset DISPLAY completely.

The IDL Virtual Machine which lets you run an IDL “executable” file without the need for licenses. Here's some more information about this feature from NYU Physics: http://www.physics.nyu.edu/grierlab/idl_html_help/distributing6.html

This can also help you run multiple IDL jobs in parallel through condor without being encumbered by available licenses. See http://www.iac.es/sieinvens/siepedia/pmwiki.php?n=HOWTOs.CondorAndIDLVirtualMachine for some pointers.

IDL 7 online help may not work properly without taking special steps. Their release notes have this to say about the fix to their help system:

  If you use the IDL Workbench, simply starting the Workbench
  with the

    idlde

  command causes IDL to use the newly-installed version of
  XULRunner. If you use IDL in command-line mode (without starting
  the IDL Workbench), you must start the IDL help server before
  starting IDL in order for the ? command to work properly.
  To do this, enter the following commands at the shell prompt:

    idlhelp
    idl

  Note that the IDL help server continues to run after you have
  ended your IDL session. If you prefer to shut down the IDL help
  server, enter the following command at the shell prompt:

    idlhelp -command shutdown

  Depending on your system, it may take several moments for the
  help server to start. Leaving the help server running speeds up
  the response time of the ? command significantly.