A great blog I read regularly "answers" search queries with funny answers and (as usual for them) drawings on a dry erase board. I'm going to start doing the same thing, without the funny answer or drawings.

idl list
IDL added lists (and hash tables) in IDL 8.0. If you don't have IDL 8, check out my implementation which I was using before IDL 8 (IDLdoc still uses it). There's also a hash table implementation and some other collections.
idl 8.3
Yes, IDL 8.3 came out in December.
mpi idl
I am the product manager for FastDL, a suite of parallel computing tools which includes a library of MPI bindings for IDL.
idl flow visualization
I hope you found this overview. I also have some IDL code to implement LIC (line integral convolution) visualization.
sublime text idl syntax
I'm not sure, but I have heard that Sublime extensions are similar to TextMate bundles. I have a TextMate bundle for IDL.
latex in idl

Historically there have been multiple ways of inputing LaTeX into IDL graphics, but there is a nice way built into IDL:

IDL> print, tex2idl('$e^{i\pi} + 1 = 0$')
e!Ui!Mp!N + 1 = 0
IDL> xyouts, 0.5, 0.5, tex2idl('$e^{i\pi} + 1 = 0$'), /normal, charsize=3, font=1

This should produce something like:

This same notation can be used in IDL's function graphics routines:

IDL> p = plot(findgen(11), title='Title is $e^{i\pi} + 1 = 0$')
"idl -e" option

There is an -e option to the IDL command line which executes a string as an IDL command:

$ idl -e "print, 'Hello'"
IDL Version 8.3, Mac OS X (darwin x86_64 m64). (c) 2013, Exelis Visual Information Solutions, Inc.