I don't know why I'm on an immigration kick lately. I think it's just a coincidence.
This article has a pretty good summary of Catholic thinking on immigration. Basically, as a nation, we have an obligation to welcome immigrants, but also the right to limit the number we accept to what we can handle. We also have the right to acculturate them to our culture and expect them to obey to our laws, which clearly entitles us to turn back illegal immigrants.
It's a good read.