(I have to warn you this post is partially serious and partially satirical. I trust you can separate which parts are which. If you can't tell which is which then feel free to ask.)
First of all, sovereign states have not only a right but a duty to protect and control their borders. People ought to enter the United States through the legal channels, and not by sneaking across the border. It would be perfectly acceptable and in the power of the United States to build some sort of fence or wall. I think it would be best if we prevent people from getting into the country illegally to begin with, that way we don't get in the situation of having to deport them, ripping them away from the life they get use to living. It get complicated if they make it here and have children born in the US, since their children will be US citizens legally here but their parents illegal and in danger of being deported. If we build a fence greatly reducincg illegal immirgation we could then have one final wave of amnesty for those already here, some 11 million I believe. But as long as illegal immigration is as strong as it is now we can't just have just one last wave of amnesty for those already here. Each time we offer amnesty we end up encouraging more people to come because they know we'll give amnesty again, sooner or later. We can't quit anytime we want to. Its unfair to those who are trying to do the right thing and enter the country legally if we give amnesty to those who broke the law.
Now before you compare it to the Berlin Wall we must keep in mind that it was a wall to trap people in an opressive state while this would be to keep people not totally out, but rather from sneaking across the border. A fence does not mean immigration stops, it just greatly reduces the illegal immigration, which is of course illegal. A few years ago a fence would not be technologically feasable. But now we could build a fence with cameras and heat sensors, etc. to keep monitor it. Furthermore, we could run fiber optics through fenses or walls so that we can know immediately if and when there is a breech in it and send border patrol to catch the illegal immigrants crossing and fix it so that more won't get through that breach. Ther's even in the works unmanned ATVs with gravity sensors to find undergound tunnels. I don't believe a fence alone prevents people from crossing, but rather allows the authorities to locate those who still try and to slow them down enough to intercept them. If we can build miles upon miles of highways crisscrossing the country north, south, east and west numerous times we can probably build a fence of some sort crossing the southern border east and west once. Some have argued against building fences on the border since they force immigrants to make more dangerous crossings elsewhere on the border. Besides the fact its a choice to enter the US illegally, if a fence is built along the whole souther border then this would no longer be the case.
Now obviously this would not stop all illegal immigrantion. Even if no one can cross the fence, people would still cross on water, which would too hard to stop altogether. But the truth of the matter is that our southern border is where most illegals cross and so where we should focus our resources. We can't be totally secure unless we were to take the technology Bush used to cause Hurricane Katrina and create storms on the east and west coast, preventing people from crossing on water. That combined with annexing Canada, Mexico, and Central America down to the Panama canal would greatly reduce the amount of land bordering other countries we have to secure, allowing America to be the first country in history to have complete control over our borders. We could even go into complete isolationism by putting a one way turn stile down south for people to leave America if they choose, but not get in. Or if we don't want to spend the money on a fence we could set up a mine field on the southern border since we haven't signed the treaty against the use of land mines. Then we could build a second Statue of Liberty down there welcoming in all who survive the border crossing. Paris has the smaller prototype, we can just go to war with France and take their Statue of Liberty, and the Eiffel Tower while we're at it. Perhaps we should just annex the whole world and then we'll have absolutely no borders to secure.
There are several other reasons for building a wall. The Roman empire built walls, e.g. Hadrian's Wall in Great Britain, so if we are to be an American empire we too should have a wall. Furthermore, China, our upcoming rival, has two walls, the Great Wall of China and their firewall. We can not allow ourselves to fall behind in the wall race by permitting a wall gap. We need a Great Wall of America. We could top them by building a southern and northern wall plus setting up our own firewall, giving us three walls and China a meager two.
I had been intending to also talk about legal immigration, guest worker, etc. but I'm afraid this post is getting rather long so I'll save that for another time. There was an interesting show on Talk of the Nation on NPR yesterday about guest worker programs in other countries which you may want to check out. NPR: Talk of the Nation: Immigration and Guest Workers