According to the previews at the end of episode 2×07 “36 Hours” of Brothers & Sisters, Luke will definitely make his appearance in episode 2×09 of Brothers & Sisters entitled “Holy Matrimony!“.

Still awaiting confirmation if he will appear in 2×08 and 2×10 (which will most likely be the last episode produced because of the writers’ strike).

ETA (Nov. 12, 2007) : It is now confirmed by ABC’s press release that Luke will be appearing in episode 2×08, “Something New,” which airs on November 25th.