Medieval Theme has a lock when I try to select it in online multiplayer on PS3.
Probably because the other player doesn't have it!?

Will there be an update for this...seems unfair if I can't use the set in online multiplayer this way?

Quote from the release:

Medieval Tales includes:
• New Campaign Missions. 10 all-new single player campaign missions set in the new medieval theme with narration scripted by award-winning writer, Dean Wilkinson.
• Medieval Theme. A stunning new medieval theme for Worms™ Revolution that can be used in both local and online matches.
• 5 New Hats. Choose from Hero, Wizard, Peasant, Guard and Executioner hats for your Worms to wear.
• 3 New Speechbanks. Ever wanted your worm to speak like a Hero, Hostage or Villain? Now they can!

So far I haven't been able to use it in online multiplayer on my PS3!

You can use it in online play... provided the other player has it too. It's additional data to download, and it stands to reason if the other player hasn't purchased and downloaded the theme that they won't be able to use it.

Therein lies the problem...the other dlc's weren't needed for the other players to have been bought, so they should include the data for this in a new update.

It's even worse, that the other players don't see my Medieval customizations for the worms, as it defaults to bare worms and original soundbank :P

I also don´t know why this theme is locked and the other themes not locked when only the host it bought.
And I am very interested in an answer why the Mediaval theme is locked when only the host it got!

Well from reading the post above yours it seems that the other two DLCs might have been included in the original software but the Medieval set had to be downloaded. Hence Medieval can't be played if only one preson has it because the other computer/console doesn't take software from the host.

any feedback from TEAM17 would be nice, concerning this issue?