XBox 360 Pre-1.2 version progression.

One month ago I bought Terraria for $6.00 after watching the price fall steadily for the last 6 months or so. Since then I've done a lot, but I'm stuck now and can't seem to progress.

I have no way to update the game, so I'm stuck with 1.1 (at least I believe so based on the wiki and other websites).

I've killed all the mechanical bosses on a single hardmode world, but nothing else seems to be happening. No event messages, no signs of bulbs or plantera. Am I missing something? Was I supposed to do a certain action between the mechanical bosses and plantera?