Genesis 3:16 - 16 Then he said to the woman, “I will sharpen the pain of your pregnancy, and in pain you will give birth. And you will desire to control your husband, but he will rule over you.” - NLT

Introduction
As punishment for disobeying God and eating fruit from the Tree of the Knowledge of Good and Evil, God made childbearing more painful and put Eve under the rule of the her husband. Christians generally agree that this is why labor pains are so sharp, and why Paul can exhort wives to submit to their husbands as to the Lord (Ephesians 5:22). However, that seems to be where the consensus ends.
Childbirth
A straightforward reading of Genesis 3 and 4 would lead one to believe that Eve's first children were Cain and Abel, born outside the Garden, after the Fall. This would not be a problem except for one word in Genesis 3:16...
“I will sharpen the pain of your pregnancy, and in pain you will give birth.
In older translations, the word sharpen is translated as increase. God is telling Eve that the discomfort of bearing children will be worse. Was pregancy already difficult? How did Eve know?
Was there Pain Before the Fall?
Many Christians conclude from v. 16 that childbirth began before the Fall. Some Young Earthers have a difficult time with this, because they do not believe there was pain before the Fall. However, God did not say that pregrancy would start being painful; He said they would be more painful. It means there was pain before the Fall, and Eve knew about it.
Was there Childbirth in the Garden?
In Genesis 4, after Cain killed Abel, he feared that others would kill him. He then found a wife. Clearly, there were other people alive, but when were they born? Were they born in the Garden, and expelled with Adam and Eve? Were they naked until the Fall? Did they sin? Did God make clothing for them, too? On the other hand, they could have been born after the Fall, but before Cain. However, that leaves the question of v. 16 unanswered.
Eve's Perfection (or Lack Thereof)
Many Christians believe the world was "perfect" before the fall, and that this excludes the possibility of death or pain before the Fall. Therefore, Eve would know about the pain of childbirth because God gave her that knowledge before the Fall. After she sinned however, God removed her protection from the pregancy's discomforts, and even increased it beyond what it would have been. God could do this, but then Eve did not have perfect knowledge. For example, she did not know that the serpent was lying to her in the Garden, and that she would be expelled from the Garen and punished by God for listening to the serpent's advice. Eve was may have been sinless to that point, but she was not perfect. As it turns out though, God only calls His actions perfect (Deut. 32:4); His creation was "very good" (Genesis 1:31). If Eve was not perfect, then perhaps she could feel pain.
A GeoCreationist Perspective
An Old Earth perspective provides a rather straightforward explanation for verse 16. It can be seen from the fossil record that mankind was created 250,000 years ago. It can be determined from a scripture, science, and history that Adam was born and/or put into the Garden around 4336 BC. It means death was in the world before the Fall, that pregnancy and childbirth already existed, and they were painful. Coming from the world, Adam would know that, and he would have passed that knowledge on to Eve, much as he passed on to her God's warning about the Tree of Knowledge.
Conclusion
To accept that pain could be in a sinless world, consider that according to 1 Corinthians 15:56, death is neither sin's sting nor its strength. It is just the opposite: sin is the sting of death and sin gets its strength from God's law. It means death was a fact before sin, before there there was a law. Acknowledging that, the idea of accepting pain before the Fall is a small leap. It even explains Cain's fear that he would be killed, not to mention his finding a wife. Indeed, childbirth was plentiful and painful before the Fall, and because of Eve's sin, God increased the pain.
1/30/12 - Added reference to post on Adam's birth year.