Internal cabling on both road and mountain is beautiful to look at (the absense of) but a nightmare to work on. It is the storm brewing that has not quite hit consumers since many have not had the necessity to work on their own internal cables yet. It basically ratcheted up the difficulty level of bike cabling by about 100-400% in our estimation. The range is determined by the how well the frame manufacturers solved the installation problems. Also, if hydraulic cables are involved, that adds more frustration factor.
But here are two enlightening videos on this daunting task. The first is a novel way of feeding the cable through the frame using a cotton thread and a vacuum cleaner.
This second video explains the general concepts and techniques for routing internal cables.
Videos courtesy of Global Cycling Network.