Are things about to get maternally interesting on CBS' "How I Met Your Mother"? 

CBS is confirming that Rachel Bilson has landed a key role on "How I Met Your Mother." And, as happens whenever any actress -- Britney Spears, Sarah Chalke, etc -- is cast on "HIMYM," speculation is already building that this role may be the one that turns out to be the mother of Future Ted's children.

Or maybe the sister of the mother of Future Ted's children.

Or maybe the best friend of the mother of Future Ted's children.

Basically, no matter who's cast, fans are going to hope that the actress in question has something to do with the show's eponymous mother. 

So is Bilson the Mother? Or the sister-in-law? Or a future bridesmaid? Really, even if we wanted to spoil this for you, we couldn't, because we don't have a clue.

Michael Ausiello, who first broke the casting story, added to speculation with word from series co-creator Craig Thomas that Episode 100 would include "some series mother action," though that may only be discussion and clues about the mother, rather than some grand reveal.

If Bilson's character isn't introduced until Episode 100, fans have a lot of time to speculate before she arrives. This season's most recent "HIMYM" episode, "Double Date," was only Ep. 90 for the series. CBS sources say that Episode 100 is tentatively scheduled to air on January 11, 2010.

Best known for her run as Summer on FOX's "The O.C.," Bilson's feature credits include "The Last Kiss" and "Jumper." She was most recently seen in several episodes of NBC's "Chuck" as a deli owner named Lou.