As I usually never get answers here (4 questions without any answers, not even a "just search on Google..."), I just decided anyway to come and give solutions when I find some :-p
This problem/solution occures only with a new BT5 R2 installation (no need with an update).
Scripts using DHCP3-server don't work anymore while working with BT5 R1 and under.
DHCP3-server is not installed and when you try the famous
you get a message error because of bad/missing dependencies.
apt-get install dhcp3-server
Last version of dhcp3-common is ubuntu3.3 and already installed.
Last version of dhcp3-client is ubuntu3.3 ans already installed.
Last version of dhcp3-server is.... ubuntu3.2 and cannot be installed with other 3.3 dhcp3 dependencies.
Force the downgrade by....
Replace dhcp3-common and dhcp3-client by the previous ubuntu3.2 version
then install the last dhcp3-server (ubuntu3.2)
=> Works perfectly!