Apple has a technote out, regarding the fact that after performing an upgrade installation (the default type) of Mac OS X 10.5 Leopard, an administrator account may change to a standard one.  While I have not seen this, a co-worker has seen this once.

This only affects Mac OS X 10.5 (Leopard).

How to fix this?  Enable the Root user, and fix the Administrator flag.

1. Start from your Mac OS X 10.5 Install DVD. (You do not need to install Leopard again.)
2. From the Utilities menu, choose Reset Password (do not choose Firmware Password Utility).
3. Follow the onscreen instructions to reset the root password (the root account is not the same as your account). Specifically: Select the name of the drive that Mac OS X is installed on, select the user named “System Administrator (root)” from the pop-up menu, type the password in the first field, re-enter the password in the second field, then click the Save button.
4. Restart from your Mac OS X Leopard volume.
5. When the login window appears, select “Other…” and log in as the root user using the password that was created above.  Note: If Mac OS X automatically logs in, choose Log Out (name) from the Apple menu to get to the login window.
6. Go to Accounts preferences and check (enable) “Allow user to administer this computer” for the affected user.
7. From the Apple menu, choose Log Out root….
8. Log in as the user account that had this issue. (Your account should now have administrator access.)
9. Use Directory Utility to disable the root user via Directory Utility’s Edit menu. You may need to click the lock icon first).

See Apple’s Tech Note, for additional information, via Mac OS X 10.5: Administrator user changes to standard.