I played in an orchestra with a left-handed violinist, who was also a left-handed violist and a left-handed 'cellist. It is odd to see a left-handed players motions be the mirror image of his stand partner in an orchestra! You can see this left-handed violinist in the following three videos. He is tucked away on the inside of the last desk of the first violins. His head is just to the right of the tubular bells. I am afraid that the video does not show him very well and you only get short glimpses. See: http://www.youtube.com/watch?v=oDuUlO2Xs_s&list=PL066ECE40B2A16016

Thank you Martin. I even saw him on the video.

I guess the question is now why are their so many more right-handed violinists? I mean: there are more left-handed tennis-persons because they have a speed advantage against right-handed ones. Can one expect something similar for violinists or is that a cultural bias that occurs at the time of the initial training?

I assume by "left-handed" you mean that the player holds the bow in his or her left hand and the instrument in the right hand (opposite the norm). 

The "norm" is really for left-handed violin playing since the fine motor skills needed to finger the pitches are done with the left hand - but both hands are needed to play the instrument. 

The direct answer to your question is that teachers simply train all string players the same way - we don't even really ask if the child is left or right handed when they start playing. For strings, many children start playing some time between aged 3-10 - the youngest often learning to play string instruments before they can write. 

I don't know of anyone who's specifically asked if there are are MORE left-handed string players - my guess would be that there are no more than the general population.