If you run a large farm with thousands of sites it's unlikely that you will know all your site collections owners.
However, there are at least two things you should know about them.
For example, do they still work in your company, have they changed email etc?
Why is this important?
Here are a few reasons:
- Communication - Downtime, upgrades, new functionality.
- Site lifecycle - If you enabled the "Site use and confirmation" feature in WSS then this is imperative. Sites that are actively used might be accidentally deleted because the emails was sent to a user who has left!
- Charge back - If you have an internal charge model in place it would make sense to have a valid person to send then bill to.
So, what tools do we have available to make sure the site owner information is correct?
How to ensure valid email address.
If you are running MOSS, then this is not a problem. All you have to do is configure the "Profile and Import" settings in your SSP. Once setup, all user profile information will be kept in sync with the information in your AD.
See Technet for more info on how to configure this.
Things get a little trickier if you run WSS only.
You can either purchase a 3rd party tool, like Bamboo Solutions User Profile Sync or code your own solution. What's most cost effective is up for you to decide :)
Checking for invalid users
Unfortunately, there is no built-in functionality in either MOSS or WSS 3.0 to deal with "Dead accounts".
DeliverPoint offers an a "Dead Account" detection and removal feature. I have not personally used the product extensively but I assume it does what it says on the tin.
Also, once again there is the option to code you own solution. Using the classes in System.DirectoryServices namespace would allow you to check if a user account still exist in the Active Directory or not.