Upgrading Multiple Servers?

[Kurth Bemis]

We have several servers, (OK lots of servers) that we use for hosting 
websites, mail servers, firewalls, and billing and provisioning.

It's a real pain to ssh to each of the boxen and repeat the same process 
again and again to upgrade apache,php,mod_perl,mod_ssl, system files, etc.

I'm wondering if anyone knows of a tool that will allow me to create one 
"master" machine or image that can then be duplicated to the other 
machines, so that all the machines are exact copies of each other.

The config and users files are on separate partitions then the binaries 
so that's not an issue.

The solution needs to scalable, as we are adding machines every other 
week it seems.

Any ideas?
~kurth