It looks like the 80GB Metal Gear Solid 4 bundle really was "limited edition." The $499 bundle has been deleted from the Wal-Mart databases, to make way for the 80GB Core and upcoming 160GB Uncharted pack. Considering the current MGS4 pack is the only system to support PS2 backwards compatibility, this may be your last chance to get your hands on a PS3 that can play PS2 games. Neither the current $399 80GB Core nor the upcoming $499 160GB bundle will support PS2 games at all.
Stay tuned for Sony's official response.