After some days of trying i just found the solution to use my HTC Diamond within Ubuntu.
My Problem was, that the internetconnection was quite slow when i was connected through ICS (internet connection sharing) ... the solution is very simple:
just open your terminal after the ics connection is established and type:
sudo ifconfig eth1 mtu 1392
(probably you have to change eth1 to eth0 or eth2 or wherever your device is mounted)