Robert Culp, who performed many roles but is best known by me for playing Breen in Half-Life 2, died. It seems likely that his character wouldn't have returned anyway, as he probably went to the alien world and got some sort of bio-modification to live there.
Either way, Breen was the best character in Half-Life 2. The announcement from Valve describes him as one of gaming's best love villains, but that's not quite right. Dr. Breen is not a villain. He is an antagonist, but he's a good guy. A man willing to make the compromises necessary for the survival for humanity is far better and more virtuous than someone like Gordon Freeman, who would just shoot bullets at an impossibly powerful alien invasion, leading his men to a glorious but inevitable defeat.
The best part was how Breen actually called out Freeman on his crap. When he accused Freeman of building nothing, only destroying things, he made the most effective and insightful criticism of Half-Life's protagonist possible. Gordon Freeman is simply an autonomous kill-bot: far less interesting, and less good, than a man like Breen.