Found a way to install it for people who are interrested.
First I upgraded to debian 6.0 (squeeze)
Replacing the "lenny" in /etc/apt/sources.list by "squeeze"
running:
# apt-get update
# apt-get install apt dpkg aptitude
# apt-get dist-upgrade
rebooting
then I added squeeze backport to my sources.list
deb http://backports.debian.org/debian-backports squeeze-backports main contrib
deb-src http://backports.debian.org/debian-backports squeeze-backports main contrib
then ran the good old apt-get update;apt-get install bitcoind
and voila.
Now have a debian 6.0 of course but running the bitcoin daemon at least.