According to Mac Rumors, Apple notified retailers on Friday about a change to the retail box copies of Mac OS X 10.5 Leopard. The latest version of the retail box is now OS X 10.5.1 which was released on November 15th 2007.
The update was previously only available via an online update, now new buyers of Macs or those upgrading using the retail box copy will not have to run the update.