After the upgrade from lenny to squeeze, cron.daily were in defunct state every day. This causes a memory overload.
When the cron.daily starts, we can see warning in file /var/log/messages :
Sep 19 08:41:02 mx1 backup-manager: warning * When validating the configuration file /etc/backup-manager.conf, 1 warnings were found.
Even if all packages are up to date, it looks like a bug of backup-manager.
The following line should be add on the file /etc/backup-manager.conf :
Warning disappears and the cron.daily runs smoothly.