I just thought this was interesting and I hope there's more reason to this change than needing to buy a new Samsung branded MHL adapter.... According to the article below, previously purchased MHL adapters won't work on the Galaxy S3. You have to buy their new one with a new pin configuration.
Source: http://www.engadget.com/2012/06/09/galaxy-s-iii-incompatible-with-previous-mhl-dongles/
Source: http://www.engadget.com/2012/06/09/galaxy-s-iii-incompatible-with-previous-mhl-dongles/