According to tv.broadwayworld.com Tahmoh is listed as a guest star in the Season 5 premiere of Castle. Tune in September 24 to find out it they are right!
Photo credit to Tumblr user iliedtoo (see post here)
ETA: ABC has just released some sneak clips from the season premiere, and Tahmoh is in it! See the clip below, but do beware of spoilers!