TVGuide online article : "The Bite Stuff" (har) about Angel.

and from (minor casting spoilers in link)"Insider": "Alas, other former favorites are less likely to drop in. Now that Buffy again has a pulse, a visit with the heartthrob who always made it race — Riley — is a no-brainer. However, sighs Noxon, "that's a big, big maybe. It depends on if Marc Blucas is available. He's turning into a big, fancy movie guy.

"Between him and Seth Green [who played Willow's beau, Oz], we're stuck," she adds. "And forget about Eliza Dushku [aka rogue Slayer Faith] — we can't get her back."

the TVGuide "Insider" article is summarized here at Sci Fi Wire, but they also include some spoilery info about the Buffy the Vampire Slayer: The Musical episode, and it would have really been nice for SciFi Wire to have put some sort of warning in the title, or at least in the article before you get to that part. bloody bastards).