I don't mean exactly head on, more like this, which I assume is the most common form of head on crash in left hand drive countries:
---------------
|-------------|
|-------------|
|-------------|
|-------------|
|-------------|<--------Left Hand Drive
|-------------|
|---------
-||-------------|
|-------------|
---------------
-------------------------
----------|-------------|
----------|-------------|
----------|---------
-|----------|-------------|<-----Right Hand Drive
----------|-------------|
----------|-------------|
----------|-------------|
----------|-------------|
----------|-------------|
-------------------------
What I mean is you are less likely to be in the part of the car which is crushed the most if you are sitting on what is the incorrect side for a certain country.
Sorry for the awful diagram...