@BJ, that's an example of the thinking that has finally convinced me that the wall is a good idea, but more to keep people in, not out. I'm positive Europe and the rest of the actually developed world will spring for a roof.
You only need to wall/roof in a handful of cities. Washington DC, NYC, Seattle (unfortunately), San Francisco (again, unfortunately) and the posh sections of Los Angeles should be sufficient. You might even get the rest of NY State to pitch in on walling-off NYC.