In what could turn out to be the mother of all casting scoops, Rachel Bilson has landed a pivotal  some would say legendary  role in How I Met Your Mother’s 100th episode.
Last month, exec producer Craig Thomas told me that the milestone episode (airing in January) would feature “some serious mother action,†adding, “It’ll be the closest Ted’s ever come to [discovering who she is]. He’s getting a step closer every episode this season, and episode 100 [we] kind of go bananas with it.â€
Thomas went on to say that he and fellow exec producer Carter Bays have been “spending a lot of time trying to cast the female role in Ted’s story for episode 100″  a role a Mother insider confirms has gone to Bilson.