I am far from an experienced climber, but this thread piqued my interest and off I went to research. Here are some links of interest I found:

The matter was discussed here at ETS nearly 10 years ago.

New England Ropes offers what they call "Micro Rappel System". It is worn as a belt that can convert into a harness and contains 82 feet of 5mm "tech cord" along with other climbing gear. They claim it was designed for U.S. special forces. (No affiliation)

From what I'm reading, it appears that special material 5mm cord and even some 3mm is used for emergency rappelling in the military and by firemen, but it is considered a one-time-use item because it is overly susceptable to wear.This thread over at backpackinglight has been informative. No affiliation.

Again, I'm the farthest thing from an expert in the matter; I just like to read.