Why might the title be A Doll House instead of A Doll's House?

Nora isn't the only doll in this house because she has made her children dolls as well. When they return from playing outside (right before Krogstad appears for a private visit with Nora in Act 1), she calls Emmy "my sweet little babydoll" and all three of the children "my pretty little dollies." Nora takes off their coats and hats as though she were undressing dolls and then proceeds to play hide-and-seek with them as if she were playing with dolls herself.

Nora has little idea of how to be a mother, having had only Anne Marie to serve in that role. Now Anne Marie continues in that position with Nora's children. Is Anne Marie also responsible for making Nora a doll? Her stunted personality development is typically attributed to her father and Helmer, but I think the nurse has a role as well. 

Why might the title be A Doll House instead of A Doll's House?

Ah I see, that's a good point. Is Nora a doll or not? This may be the central idea behind the story. Either title could work but the one you suggest "The Dollhouse" is more of a stark contrast, showing that Nora, who is not a doll, must escape living like one.

Why might the title be A Doll House instead of A Doll's House?

Personally I don't see that there's a real difference between the different intepretations of the title. Is there a material difference between "A Dollhouse" "A Doll's House" or even "The Dollhouse"? It doesn't affect the meaning, I don't think.

I'm with you. If someone does have an argument for why there is a difference, please enlighten us. This discussion is very interesting.

Why might the title be A Doll House instead of A Doll's House?

This is a good point, and one I've addressed before, but here is my understanding.  The title has been translated as both "A Doll House" and as "A Doll's House."  I've even seen it translated as "A Dolls House," though to me this is the poorest interpretation. In any case, the original Norweigan is difficult to translate and a choice is made on the part of the individual translator.

Your take is certainly valid, but if you argue the other side, you might postulate that the possessive is true, that Nora lives in a superficial, closed world, treated as an object rather than a real person. 

When in A Doll's House do you understand the title's significance?

The title of this play makes reference to Nora's position in her own household, as well as Nora's perception of the world.  Not only have the men in her life treated her like a "doll", to be dressed up and played with, but she herself has lived as that doll.  She has played into the role, and she has always viewed the world from only the perspective of her little house.  This is why she does not understand the seriousness of the fraud that she committed, and why she doesn't understand the situation of either Krogstad or Mrs. Linde.  Her house is her own little world.

The action of the play works to pull Nora from her single-minded perspective and from the role she has played.  There are a few hints to the title's significance in Act I and II.  For example, in Act I, Nora calls her own daughter "My sweet little baby doll!"

The moment where the title is revealed comes in Act III, however, after the conflict with Krogstad has been resolved:

• NORA: When I was at home with papa, he told me his opinion about everything, and so I had the same opinions; and if I differed from him I concealed the fact, because he would not have liked it. He called me his doll-child, and he played with me just as I used to play with my dolls. And when I came to live with you—

Nora finally understands how others have viewed her, and is on the brink of establishing her independence.

When in A Doll's House do you understand the title's significance?

Great question for the discussion board!

For me, it is clear why this play has its title from the moment it is apparent how Torvald treats Nora.  It is clear early on how Torvald views Nora.  She is treated with kid gloves and is not taken seriously.  Torvald belittles her and doesn't listen to her feelings, wants, and desires; therefore, she resorts to doing many things without his knowledge.  In the end, she simply cannot take any more of it and leave Torvald and her children behind for a new life.

At what moment in A Doll's House do you understand why it has been given its title?

The problem with this question is that having read, studied, taught and thought about this play for a long time, I am able to pick up references to the text that might suggest the central governing image of Nora as a doll in a doll's house much earlier than this reference is made explicit. Therefore, consider the following quote from Torvald at the beginning of the play:

That is like a woman! But seriously, Nora, you know what I think about that. No debt, no borrowing. There can be no freedom or beauty about a home life that depends on borrowing and debt. We two have kept bravely on the straight road so far, and we will go on the same way for the short time longer that there need be any struggle.

Torvald is treating his wife as if she were a child, and as it is clear Nora is the protagonist, it does not take much of a leap of imagination to look at the way that Torvald treats his wife to see his control and treatment of her is reminiscent of an adult with a child.

However, if we want to look at explicit references to dolls and doll houses, consider this quote from Act III when Nora finally confronts both herself and her husband:

But our home has been nothing but a playroom. I have been your doll-wife, just as at home I was papa's doll-child; and here the children have been my dolls. I thought it great fun when you played with me, just as they thought it great fun when I played with them. That is what our marriage has been, Torvald.

If there had been any doubt before, the room and the marriage of Torvald and Nora is explicitly compared to a doll's house and dolls. The unreality and illusory nature of Torvald and Nora's marriage and life is thus made explicit and the reference to the title is definitely explored.