I was installing Server2003 Std and Server2008 Std on hardware recently. When joining the servers to the domain during the install, I was asked for credentials.
I answered with the the DC's Admin user and password.
What minimum level of credentials are normally required to join a member server to a domain?
By default a domain user can add up to 10 computers to a domain. There is a way that administrators can restrict this default behavior to only allow Administrators to add computers to the domain, by using ADSI Edit.
http://support.microsoft.com/kb/243327
By default Authenticated Users are assigned this privilege.
This is defined by a GPO:
Computer Configuration --> Policies –-> Windows Settings –-> Security Settings –-> User Rights Assignment --> Add workstation to domain
As noted by @redknight, by default users (excepting domain admins) are able to join 10 computers to a domain.
"Add computers to domain" is not restricted to Domain Admins at all, that wouldn't be very good security. There is also no distinction between adding a workstation, or adding a member server.
This article describes some of the details.