The SMF License Agreement prohibits altering the copyright notice displayed at the bottom of your forum. All copyright notices within the source files, and as generated by the Software as output, are retained and are unchanged.
Altering the copyright notice will terminate your license to use SMF, and may result in a warning message at the bottom of your forum.
The SMF version number is considered separate from the copyright itself, and altering it is not considered a violation of the license. However, altering the SMF version number may make it more difficult for the SMF team to offer appropriate help tailored to your version of SMF.
If a warning message is visible, it is likely that something has been done that has altered the copyright function. If the administrator is seeing these messages, they should check the mods which they recently installed and uninstall them as soon as possible.
Outdated language files can also cause this message to show. If that is the cause, the admin should visit the Downloads Page and download updated language packs.
The SMF copyright must remain. If it is removed, Simple Machines reserves the right to seek legal action. You cannot alter, remove, or hide the copyright statement at the bottom of any SMF forum in any way, with the exception of having written permission from Simple Machines.
Also, you are not allowed to alter or remove any credit information from the SMF source code. You cannot copy any part of the SMF source code, or claim any SMF code is yours.
There are a few reasons why the missing copyright message might be showing:
Editing the copyright message - You removed required information from the copyright string. This is most likely a violation of the SMF license agreement.
Theme incompatibility - Installing a theme intended for 1.1 on 1.0, or 2.0 and vice-versa, will most likely cause the message to appear. Consult the author of the theme to make it compatible with your theme or use a different theme that is compatible with your version.
In process of upgrading - If you are upgrading from some version of 1.0 to some version of 1.1, you might encounter this message as the files upload. Once all the files have been uploaded, and upgrade.php has been run, this notice should disappear.