Ways to remove android device encryption
I did manage to turn my device back to an unencrypted state. If you do a device backup via the adb backup command you get a nice .ab file. There exists at least DroidExplorer that can convert these backups into .tar files:
How do you extract an App&#39;s data from a full backup made through &quot;adb backup&quot;?

I'm not realy sure why there are some apps that did not get backed up but I think this migth have to do with the market license stuff. Only some bougth apps were not backed up. If you want to make sure that all your important apps got backet up use DroidExplorer to check it out.

Then you have to perform a factory reset in order to get it back in an unencrypted state. Now you can eigther use adb restore or Titanium Backup(which also allows to only restore some data) to restore the backup.

Even if this is an answer to my question i will not mark it as one for now since I'm still convinced that you could also load a custom software via fastboot to encrypt the data partition without a factory wipe.

I also found a tecnical description from google: http://source.android.com/devices/tech/encryption/android_crypto_implementation.html