Quote Originally Posted by Uesugi Kenshin
One reason left handed people were thought to be untrustworthy is that since you shake with your irght hand you couldn't stab the other person or pull a sword on them, but if you were left handed you could easily stick him in the gut.

I don't have evidence of this but I was told this by a reliable person.
I have heard a slight variation of this idea which states that shaking hands with the right hand evolved as a gesture of good faith. By presenting ones empty right hand you openly declared that you were not carrying a weapon in it, and by mutally clasping each others right hands you were actually preventing each other from drawing a weapon.

I recent used this idea in a roleplaying game I run.