[Update: SOE dropped us a line to let us know that there were some last minute changes to their press release which failed to make it into the announcement we received. The "Hard Mode" option for HIVE Alert is not part of this update. We're sorry for any inconvenience.]
Nope, this is not the paid DLC, "Fight for the Light," that was announced a few weeks ago for DC Universe Online. "Fortress of Solitude" is part of the regular free update packs Sony Online Entertainment is providing. The new content includes a whole new mission where you fight alongside Superman and Lex Luthor to defeat Braniac in the Fortress of Solitude, new gear & loot and the first PvP deathmatch arena.
The update will also add a "Hard Mode" to the existing Batcave Group Combat Raid and HIVE Alert. The content is available today for both the PlayStation 3 and PC versions of the game, so go ahead and give it a shot.