I just finished chatting with Joram over at www.ringbearer.org. He was telling me that he has recieved an e-mail concerning who will play Merry. Joram did say that the rumor is unconfirmed, but he is pretty certain of its validity.
The rumor is that Merry will be played by Dominic Monaghan. The actor is 22 years old from Manchester, England. He has been in several things. Most notably is his appearance on a PBS show called Mystery! The above picture is from that show. Dominic is now several years older than he appears in that picture.