I'm a left handed person and was brought up to use the fork in my left hand and obviously I've got used to that but it does sometimes feel unatural and I do occaisionslly pick them up the other way round.
When I look at the way a right hand drive car is layed out I often think it was designed for left handed people, as most of the controls, gears etc are on the left.
I think a lot of it though was borne out of superstition rather than table manners, the devil apparently is always depicted as left handed, so it made sence in the dim and distant past to convert to the fork in the left hand to avoid being accused of witch craft and end up being burnt at the stake.
Driving on the left, I'am told, all go's back to the days when it was advantagous to carry your sword in the right hand, (as most people are right handed) when walking down a road, the idea being that you could better defend yourself if you should meet the enemy coming the other way.
Sorry for going off at a tangent, thats another thing lefties tend to do.