Months ago rumors began circulating about packs that were for a split set, and speculation ran rampant as players tried to figure out if it was true, and if so how it would be handled. Today Monty Ashley from Wizards revealed them on the Wizards site.
This is something that's happening only at the Mirrodin Besieged Prerelease Event. Players will receive three regular Scars of Mirrodin booster packs and their choice of three Mirran boosters or three Phyrexian boosters. This is something we’re doing especially for the Mirrodin Besieged prerelease; regular Mirrodin Besieged boosters will be, well, regular.
Also of interest is that there will be two Pre-release promo cards, one for each faction.