I have arrived with a +5 sword of stinging truth!
I am sorry to say to the two of you in here fighting for the validity of this release, but I have come to the conclusion there is no way this release is factual. In fact, I did so by using the very box art pics you said were so similar. To begin with I noticed that the "The Pokemon Company" logo looked a little fishy on the Opal box, so I opened up some comparison tools and the logos when blown up a bit look nothing alike. To begin with the actual logo is quite a bit larger, but that isn't even the most important thing. If you look closely the accent on the e on the Opal box points upwards, whereas the actual logos accent has a slight curve towards the horizontal. I highly doubt they would change the logo, ESPECIALLY if they were created at the same time.
Secondly I checked the "Game experience may change during online play." This is standard and the same on all DS boxes. This one you can see better if you don't blow it up. The C on Change is closer to a slightly curved [ which is true for the Diamond box. However on Opal it is a perfect curve just like C. Same story with the G on Game. In other words, they were written in DIFFERENT texts, not the standard.
Finally I would like to point out that all of the letters that spell OPAL are also in diamOnd and PeArL and appear to me to be copy and pasted from these boxes, the Os glow altered, slightly rotated and put back together.
In other words, a well put together fake. But a fake nonetheless, unless someone can point out a good reason why the fonts would be changed on stuff where it shouldn't be.
Furthermore, the third game will be called Topaz. Duh! :P