The US is a vast country. We have over 12,000 miles of coastline and borders with Mexico (1,900 miles) and Canada (5,500 miles--including Alaska). A wrinkle in the immigration discussion is securing our borders from terrorist threats. For serious security hawks, this is a completely distinct discussion from what to do about illegal immigrants coming here to work. (Racists may conceal their concerns behind the language of security, but real hawks care little about whether workers are sneaking in for higher wages.)
It's reasonable to ask the question of how secure our borders are and what we can do to make them secure. I'm far from a hawk, and yet it seems pretty obvious that it's possible to walk undocumented into the country from a number of locations. It's also clear, given the size of the country, that shutting down the border physically just isn't reasonable. Most of the solutions I've heard about seem to ignore these mathematical realities, and offer the usual ineffective efforts to stop entry: more border patrol agents, larger detention halls, technology solutions like unmanned vehicles, even the suggestion of a great wall of immigrant repelling.
Before we embark on that folly, though, we need to ask what our real intention is. Are we going to actually secure the borders, or just do a good job of looking like it so illegals can scamper in to do the work we've carefully held open for them? We have a schitzophrenic relationship to illegal immigrants, and as long as that exists, we'll never have a secure border.
Second, we have to have far better systems in place to deal with immigrants who do manage to arrive illegally (if that's the intention). Suggestions here include serious workplace enforcement, better systems of tracking individuals across local, state, and federal jurisdictions, and coherent laws to deal with violations.
I would like to question the notion that we can secure our borders, though. We can't. We will always be subject to attacks through borders more porous than we'd like (I haven't even bothered to mention our ports, for example). We must find a way to balance reasonable security measures with a mature understanding that the US is a target for terrorists. Invading other countries, torture, secret rendition--these things make us an even bigger target. A militaristic solution won't work and, worse, it undermines safety by giving a false sense of security.