Unlike the Novell Client for Windows XP/2003, this value is None (value not present in registry by default) in the registry. By default, the username field in the Default System Profile is not editable. This is a design-level behavior that differs between the Novell Client for Windows XP/2003 and the Novell Client for Windows 7.
On the Novell Client for Windows 7, the eDirectory username /inside/ the profile is implicitly the same as the user for whom the profile has been created and/or is about to be created. The "Username:" field in the "Credentials" tab of the Windows 7 client is grayed out because it will by definition follow/match/mirror the username that is entered into the Novell Client for Windows 7 credential provider.
Allowing the eDirectory username inside the profile to differ from the eDirectory username entered into the credential model would break the overall intended and expected behavior. The username entered into the credential provider is what selects which profile might be used. (i.e. First check for a user-level profile for the user entered, else fall back to a system profile.) The profile selected then provides everything besides the username itself.