Here's what I have to say: Castiel doesn't need to apologize for anything.
It's not that he's not responsible, they all are in many ways, but he's practically proven himself multiple times in redeeming himself from those past wrongs ever since the fourth season finale. He rebelled against Heaven for humanity, he died in protecting the Winchesters, he fought on their side, he killed his brothers and risked himself over and over again, even in times of doubt and despair he still fought alongside them and what they believed in, even risking losing his grace for them. Isn't that enough in convincing that he's making up for those things he wanted to do, but couldn't, based on the upper hand in Heaven? I think many people are forgetting that regardless of what happened, everyone was a pawn to ensure the Apocalypse and the unleashing of Lucifer happened. I mean, surely Castiel could apologize for letting Sam out of the panic room, but what would that accomplish? It would be moot point right now considering all what he's done for the boys since then, and quite frankly I believe actions speak louder than words.
Some people are still hung up on Castiel letting Sam out of the panic room dealio, and while yes bad, but if Castiel hadn't done it another angel would have. It was an inevitable incident that, once again, they all played their part in no matter what. It's almost like people complaining about Castiel "manipulating Dean" in the very same episode. Never mind that those were his orders that he clearly was reluctant on obeying, but had to considering he was held on a very short leash by his superiors. Castiel was torn between wanting to do the right thing and following orders, which he knows the consequences for disobeying; he was growing and learning, he was afraid and rightfully so knowing the manipulative ways Heaven operates. He clearly had remorse, he was tormented with having to do those things but it was like Heaven was placing a gun to his head forcing him to otherwise dire consequences would occur. He had no choice, until Dean convinced him that he did in the finale. Remember, Castiel (and many other angels in fact) weren't given the inside scoop about the true motives the higher superiors were up to regarding the Apocalypse, hence why they were all given to believe they were doing things for the good, until Castiel had discovered it was not the case, which led to him being "retrained" and punished in Heaven and doing those things that he did.
It also reminds me how angry I was at Anna when she came back in S5 to blame Castiel for letting her get caught when, hello, wasn't she the one who previously mentioned that if Castiel had gotten dragged back to Heaven unwillingly it was bad, really bad? Never mind that he did tell her that she shouldn't have arrived, so she should have understood the circumstances he was under despite all of that. She should have known better, especially if she was a so-called "former superior" and all, but no. She blamed him instead of herself in that matter. Another reason why I dislike her character, but that's neither here nor there at this moment.
So in conclusion to this argument, was Castiel responsible for some things happening? In ways yes, as they all were. But he shouldn't need to apologize when he's already made countless amends for everything through his actions since all of that. Think of all he's done for the Winchesters, for humanity, what he's sacrificed and gave up. People should be focusing on what he's done rather than dwelling on a previous incident.
Just needed to get that off my chest.
Other than that, it appears that Creation Entertainment has really pushed the Miracle Button because, come January 2011, Supernatural will be coming to San Francisco! Guys, this is literally right next door to me. \O/\O/ The main reason why I initially went to LACon was because it was the closest convention there would be without having to travel all the way across the country to go to, and lo and behold they've put them even closer. I didn't even realize it until someone on my flist mentioned it a few days ago. This is just awesome. Although come to Sacramento next, will ya guys? The only convention here is Twilight-based and that's kind of insulting, really. We need more variety.
Guess who might be going now that the offer is right in my backyard? ;D