Are there any significant differences in the rights, privileges, permissions, or other powers held by the built-in Administrator account, and those held by non-built-in Administrator accounts?
Some applications are incorectly coded to look for the SID of the well known Administrator account. This was an issue in the early days of NT4 but don't think it's an issue now...
I don't think any applications match on the name "Administrator" since that is Engish specific and is a very poor programming practice. "Administrateur" and other language matches would have to be accounted for.
Instead applications should match on the SID of the local group Administrators
One small difference, is when a folder is created by any administrator, the default owner for that folder is the Administrators group.
This is not so much a difference between administrators and Administrator, but a possible effect that might possibly have other consequences.
Also note that this default is configurable, and anyway it is no longer supported by Vista/7/2008.