I agree, I'm pretty sure Claire's dead. Why e;se would she have left Aaron and gone with Christian - who's also dead.
And, as for people who say she couldn't have died becase she spoke to Sawyer and Miles and was holding Aaron, remember that Christoan has also spoken to characters (Locke in the cabin and the chamber with the Frozen Donkey Wheel) and has held Aaron.

I'm just wondering if Claire knew is she was dead or not, or if she'll be back at all.

I think she will be back for sure she is one of the origanal cast members i think if we were never going to see her charcter again she would have got a better send off. Another reason why i think she is dead is because Kate had a dream about her we know from past experiance that the dead appear in dreams such as Yemi and Horace I think Miles knew she was dead because he can see the dead i think thats why he was taking such an intrest in Claire after the house explosion not because he fancied her as Sawyer thought but because he knew she was dead. I also think Claire is the one that took Aaron back i think she is the only person Kate would let take him.

i agree with this theory:)))