Smealum, the same hacker who created the homebrew Portal game for DS, claimed responsibility for the hack, and also revealed Mega Evolutions for Latios and Latias. While the two Dragon-type Pokemon were previously differentiated by Latios being blue and Latias being red, it seems that they are more or less indistinguishable from each other when they Mega Evolve. The Mega Evolution for Latias is pictured above.
Don't rush off to trade your Pokemon just yet, though. Smealum wrote on Twitter he does not plan to release the hack or any of the newly-discovered Pokemon into the wild. Smealum wrote that while it would be fun "in theory," he didn't want to "ruin" the online Pokemon X / Y experience.