Arduino + Ethernet Shield hangs after some time
UPDATE

Here is a working solution testet with Mega (1.0.1) and Due (1.5.3)

Ok, it seams there is a bug in the arduino 1.0.x libraries that causes the ethernet module to hang after some time, when you have to permanetly query a server.

With the Arduino Due (arduino 1.5.2) it is now running and also much faster! So its maby the best option to use this when you have to deal with much network traffic and many operations. I will contact the arduino developers about this issue and update this answer when I got more information about this problems.