Wow thanks for all the input!

The idea with the Flash Shoe is great, with your information about the "all time 5V while taking picture" the problem really is pretty much solved.
Sadly my Hardware still didn't arrive.. crap!

Maybe some one can still explain to me, why is it complicated to send out information via USB - what has to be done to get that working? 

Hey, thank you very much about your answers. The link was helpful to me!
Im owning a Canon 550D. Sorry for not writing this.

About the implementation idea:
Magic Lantern (the firmware on the camera) has a built in intervalometer function. Using this function with an extra for controlling the arduino. So after sending the shutterEvent, wait the seconds set for exposure plus some delay and then move the slider.

About the why question:
The Dollyshield is awesome, but it is not very cheap if you buy it as a complete thing. For the DIY-Kit you need some advanced knowledge in that stuff and equipment. If you got that done, you have no housing for the arduino and its shield, the display might be okay for setting those things up, but the camera display is much more comfortable and you get to use the D-Pad. 
Im intrested in learning to program those things and thought this might me a great project.

Thanks for your great work on MagicLantern. It makes a Canon Camera so much better.

My Arduino still didn't arrive.. hopefully he comes this week.

The most burning question is, how can i send commands via USB to the Arduino in Magic Lantern?

Thank you very much for you help, i hope you understand why i like this idea.

Hey, my project intention is creating a camera slider for great timelapse videos.

On pretty much every page i found about controlling the slider with an Arduino, the camera is controlled by the Arduino (like here: http://openmoco.org/node/212). 

The idea is now to do a reverse thing, let the camera control the Arduino and use the monitor of the camera to set it up. Also the synchronisation between camera and slider can be very precise this way (shutter events and stuff). The custom firmware on the camera is called Magic Lantern (http://magiclantern.wikia.com/) and is based on DryOs, as far as i got that right.

So I ordered an "Arduino Duemilanove" to create a pattern in Windows on how to control the motor for the slider.

First of all, is it possible to send those commands from the camera via USB to the Arduino?

Maybe there are some other people out here intresting in creating this...

Really glad for every answer I get

Thank you,
subsonic