Initially when you install SharePoint, the farm administrators group will contain the group BUILTINadministrators, which is the local group on the server for administration. This in turn will usually include you domain admin group, effectively giving all domain admins full control of your SharePoint environment – in many cases this may not be desirable.
The group can be updated by going to SharePoint Central admin > Operations > Update farm administrators group
In SharePoint 2003, resolving the domain administrator having access was far more complex as a patch had to be installed.
jennye