Josh Marshall has a good post outlining some of the legal issues around the birthers' bizarre idea that Obama was not born in Hawaii and is therefore not the legitimate president. Read the post for the specifics, but it has something to do with the legal requirements for a child born to one US citizen and one alien parent being considered a natural born citizen. I had no idea it was this complicated, but, according to law, if Obama was, in fact, born outside this country, he would not qualify as a natural born citizen. It's quite complicated. The birthers' argument, presumably, is that his parents forged his birth certificate to show him as a natural born citizen because, under the law, he would not have been.
There are a couple of obvious problems with this. First, it presumes that the Obamas knew exactly what law governed American citizenship in 1961. But we're talking about an 18-year old woman in Hawaii. In this age of the Internet, it's easy to find that kind of thing out. Back then, it would have been much more difficult. Of course, Hawaii was a new state, so issues of citizenship were probably much talked about. But it's still weird to contemplate an 18-year old woman having the foresight and knowledge of American citizenship law to figure out how to circumvent it.
The second problem is that it presumes that Obama's mother and father would have wanted to prove that he was a "natural born citizen." Of course, it would have made it easier for him to be a citizen later in life, but he would have presumably had a very good chance of becoming a citizen, given that his mother was American. But the only job that I know of that requires an American be a "natural born citizen" is president. Planning for your biracial child born in 1961 to become president in 2008 would be like planning to pay for their college education by winning the lottery.
By far the most serious problem with the whole birther argument, however, is this: it assumes that Obama's mother was not present in the United States at the time of his birth. But she was 18. She presumably had no income of her own, or very much money of her own. As far as I know, she was living in Hawaii because that's where her parents lived. Obama's father was there for grad school. So both Barack Obama's mother and father were living in Hawaii before he was born, when they were married, and after he was born. Hawaii is literally thousands of miles from any other country. It would have been physically difficult, not too mention financially extravagant, for the two of them to travel to any other country, let alone Kenya, before Obama was born. They had every reason not to leave Hawaii: that's where her family was, and the medical facilities were presumably better there than in Kenya in 1961.
So here's the question for the birthers: why, in 1961, would an American woman with very little money, married to a graduate student with presumably not much more money, travel to a foreign country, thousands of miles from her family, to give birth to a child where she didn't have access to the latest in medical care, and then return thousands of miles to her parents' hometown with her husband and son? She would have absolutely no reason to do so, and every reason NOT to do so. So, unless we see concrete evidence proving that Obama's parents did exactly that, the idea that Obama was born anywhere other than Hawaii makes no sense whatsoever.