Thanks to the new backian kit for debian , here's how to install PostgreSQL 9.0 on Debian lenny:
Add the following to /etc/apt/sources.list:
deb http://backports.debian.org/debian-backports lenny-backports main
deb http://backports.debian.org/debian-backports lenny-backports-sloppy main
Then run these 2 commands
apt-get install -t lenny-backports postgresql-common
apt-get install -t lenny-backports-sloppy postgresql-9.0 postgresql-client-9.0 postgresql-contrib-9.0
, lenny-backports-sloppy backport suite - , , .