I've run apt-get update, aptitude update.
This is a fresh install, so /etc/apt/sources.list should be default, but here is mine:
#
# deb cdrom:[Debian GNU/Linux 5.0.2 _Lenny_ - Official i386 DVD Binary-1 20090628-15:08]/ lenny contrib main
deb cdrom:[Debian GNU/Linux 5.0.2 _Lenny_ - Official i386 DVD Binary-1 20090628-15:08]/ lenny contrib main
deb cdrom:[Debian GNU/Linux 5.0.2 _Lenny_ - Official i386 DVD Binary-2 20090628-15:08]/ lenny contrib main
deb cdrom:[Debian GNU/Linux 5.0.2 _Lenny_ - Official i386 DVD Binary-3 20090628-15:08]/ lenny contrib main
deb cdrom:[Debian GNU/Linux 5.0.2 _Lenny_ - Official i386 DVD Binary-4 20090628-15:08]/ lenny contrib main
deb cdrom:[Debian GNU/Linux 5.0.2 _Lenny_ - Official i386 DVD Binary-5 20090628-15:08]/ lenny contrib main
deb
http://security.debian.org/ lenny/updates main contrib
deb-src
http://security.debian.org/ lenny/updates main contrib
deb
http://volatile.debian.org/debian-volatile lenny/volatile main contrib
deb-src
http://volatile.debian.org/debian-volatile lenny/volatile main contrib