Hi!
I ran the game in 1280x800 on the previus macbook (Intel 950) with reduced graphics options. However, I only played some characters until level 20, so I dont know how later gaming in bigger dungeons works out. Some months have passed since playing LotRO and so I dont remember details, but I also dont recall ever thinking "omg this is bad/unplayable". My unliking of LotRO then developed during playing due to the characteristics of the game, but not because of the macbook's or the game's performance.
I also play WoW on this macbook and get 15-20fps usually, and about 5-10fps in large fights in MH / BT (and still use some raid-needed addons).
So I think the newer X3100 macbooks will do even better. Although I wouldnt recommend a macbook for gaming it has sufficient resources to play these games.