Found out some few nice information for my setup now... 
The trigger pin on the canon flash is 5 volt to ground.
i guess the camera hot shoe is optoisolated from the camera sinse there is never a voltage on the shoe. only resistance (4-5 ohm)
So, i connected my camera to the arduino, and found out that the shoe is "high" all the time the camera takes a picture.
And that's good news
These could be used as a bussy signal to the arduino 
and what the arduino do when low... well, that's up to me
Hope these information is nice to know for more than just me.