Merle Dandridge, the actress behind the face of Alyx Vance has posted a picture of herself in full mocap onto her Twitter page with the caption "Lovely Vera opening last night. And a very mo cap mornin' to ya!". While there isn't any indication of what this motion capture work is for, it doesn't seem like too much of a stretch to jump on the Half Life 3 hype wagon seeing as shes done the voice acting and face work for Alyx Vance for every single Half Life game released so far.
Another reason to jump on the hype wagon is the response from Ashley Johnson, the actress playing Ellie in the Last of Us for the Playstation 3. She reposted the image posted by Merle with the caption "Alyx Vance #HL3 #insidescoop" Again, not a confirmation by any means, but certainly reason enough to get your undies in a knicker. You can view the image and Ashley Johnson's tweet above and below.