I mostly agree with Matt Yglesias on the general question of immigration, but the exercise in seeing how many people the US could hold if everywhere (but Alaska and Hawaii, I guess) had the population density of New Jersey reminds me again how NY-provincial Yglesias can be sometimes. I'm very proud of my native Arizona, but really, it's hard to picture very many people wanting to move to large parts of AZ, NM, NV, and UT...and plenty of California isn't exactly human-friendly, either.
Now, granted, you can keep getting the density of Arizona to increase by packing more and more people into Phoenix, and there doesn't seem to be any limit to that process. Except for the big one that applies to all the places I'm listing here: water.
I know, it was just a fun exercise...and who knows, as demand increases perhaps someone will figure out the technology fix that'll allow 30M or 40M to live in the Phoenix metro area without the whole thirst problem (it would take over 100M to match New Jersey's density). And as I said, I agree with the larger point. Couldn't let this one pass, though.