I thought he was excellent in 3:10 to Yuma, although Russel Crowe was the big standout in that film. But I think Bale had the more difficult role to play and it was very different in that he's not this macho tough guy, but a one legged rancher trying to make a living for his family.
As far as TDK, guys I know this may sound sacreligious, but outside of Heath that movie is average at best. The movie really has no character development, it's more of a character study, an experiment. Heath made the movie interesting, and worth watching. Bale had much less to do than he did in Begins, he was pretty much an afterthought.
As far as TS, I said it before on the other thread. It's pretty obvious Bale thought the movie sucked, and his performance reflected that. He was shockingly bad, to the point it was embarassing, but part of that should fall on McG.
I don't think he's lost steam, I just think he's been in movies recently that have not taken advantage of his acting ability.