hi Chaos,
Welcome to the forums!
In order to protect our trademark, and insure that users don't abuse our intellectual property, users are not permitted to alter or remove any of the logos/text.
Although other changes to the code are permitted such as clamav updating, adding of zimlets, etc; your request falls out of the scope that we allow under the ZPL (
http://www.zimbra.com/legal.html)
If you have any questions, please let me know. Also, you can alter these things under the Network Edition.
Cheers!
john