It's really not that momentous. This is a Supremacy Clause issue, plain and simple - if Arizona had made the opposite law, one that said "we're taking down the border wall, and free ice cream and jobs for all illegal immigrants," there would be a similar challenge to preserve the total federal power over all issues related to immigration.

"makes it a misdemeanor for an *alien* to be in Arizona without carrying registration papers"

I guess this is why *the aliens* crashed in New Mexico and not Arizona. On a lighter note, commenter Alf (Alien Life Form) better cancel his summer vacation to the Grand Canyon.