I dislike being argumentative on my second post on these forums, but I have to point out that the Justice League #51 misprint is not unprecedented.
In spring of 1988, issue #5 of Wasteland was released with both the correct cover, and the cover to issue #6. The error cover is pretty well worthless, as is the rest of the series.
I know that this does not translate to the JL error being worthless, but I have to wonder if it will stay hot for the long haul, or if it is something that will cool down significantly as more copies hit the marketplace.