Friday night I was in Baltimore for the Lil' Wayne concert. I didn't actually go to the concert; I watched it with the Baltimore police. But this post isn't about what happened Friday night (you can read about that in the May issue of Governing). It's about what happened Saturday morning. I awoke late, went downstairs, and ask the doorman where in the Monument Square area I could get a bagel and a cup of good coffee. She directed me to the charming City Cafe, basically your classic urban hipster-writer hangout except for one thing: No one there had an Apple laptop. This was strange. At similar places in Santa Monica or Brooklyn, Macbooks are everywhere. But not here. I'd seen the same Mac absence in Cleveland last fall. That's when it hit me: Cities such as Baltimore and Cleveland are struggling because people there don't use Macs.   At this point, you're probably thinking: He's crazy! Macs don't make cities richer; it's just that creative types in wealthy communities like (and are willing to pay more for) Macs. Economists have a term for this error -- endogeneity. As much as I (a Mac user) hate to say it, Macs probably aren't the variable that has made West Hollywood rich. They're a byproduct of success, not the cause. Which brings me to immigrants. A favorite new(ish) theory among urban planners is that immigrants -- even illegal immigrants -- make for successful cities. Cities such as New Haven have set up support services for them. Other cities have set up immigrant officers to recruit them. But do immigrants really cause cities to succeed? Or are immigrants attracted to successful cities? That's an important question. I hope some researcher is examining it -- on a PC or on a Mac.