I don't really see what the problem is...
Do you mean you think the signature should always be at the very bottom of the post area? I disagree, traditionally a signature is a fixed part that is always inserted after the text of your post; in a way, it's part of the text, so should directly follow the text, even if the total text is shorter than any other elements (such as the profile info) influencing the height of the entire post. Anyway, that's a matter of opinion/taste, I don't think one way or the other is "wrong".
If that's not what you mean, I'm really in the dark as to what the problem is
