If you have upgraded your old Lenny boxes to Squeeze you may find that your syslog-ng is no longer working due to a change in the structure of the config file /etc/syslog-ng/syslog-ng.conf.
If your custom config changes are minor then the easy way to upgrade is to copy your config to a safe place, purge and reinstall syslog-ng:
dpkg -P syslog-ng
apt-get install syslog-ng
nano /etc/syslog-ng/syslog-ng.conf # copy back custom changes to new file format