I've done a little bit, mainly for functionality, not the "wow" factor...

I cut two holes on the side of my case to accomodate two 92mm Vantec Tornadoes, so I've got GOBS of air coming into my case now. I also drilled a whole bunch of holes through the front bezel on my case, so the 80mm fans I installed on the front of my case wouldn't be restricted.

Like I said, I was completely going for function-- All of my IDE/floppy cables are rounded for better airflow, my PSU has a 120mm fan on the underside of the unit, and an 80mm intake fan on the other side, blowing air through the PSU, also exhausting out the back. All of the Blue LED lights I have in my system just happened to be in there because of my choice of components.