Well I've thought a long time about announcing this one. I've received an email from someone who I'd definitely call "in the know", giving word on who exactly Merry will be played by. At this time I have not received any confirmation emails from Michael DeLuca at New Line or Harry at AICN. So I can only offer this up as a very strong suspicion.
In an email yesterday I got word on who was going to be playing Merry, and after an hour of searching for a picture of him I would say that's he quite an unknown! His name is Dominic Monaghan. The picture of him here is from his work on the PBS show Mystery!, and I think the pic is 3 or 4 years old. He's a 22 year old actor from Manchester.