SurferTim:
Do you have a SD card in the ethernet shield's slot? That would cause this kind of problem.
I bought an SD card and used it with the ethernet shield. Now the Udp.endPacket() is executed and I see some smiley faces on my cmd (where I use node.js). However, the loop still doesnt work. This whole thing is executed once, and returns the smiley faces once. After that my sensors get deactivated. By that I mean, my sensors have two LED's on them. One is red which is the power, and one is green which lights up only when it detects an object. After the loop is executed once, the green LED's never light up again unless I restart the shield or unplug the USB cable.