According to the official website:-
02.02.2005 : NORTH AMERICAN "BEST OF" RELEASE
The North American release of the new "BEST OF" compilation album is now scheduled for MAY 17. This release will be slightly different than the version that was released in the rest of the world in October 2004. To reflect the fact that it's a different release, there are some changes in the tracklisting, it will have new artwork and it will have a new title: ROCK OF AGES - THE BEST OF DEF LEPPARD.
For more information about the "BEST OF" album (both the upcoming North American version and the already released world version), check out
http://www.bestofdefleppard.com.
Just thought you might like to know