How good is Yale for undergrad business? How does it compare to Harvard or Wharton in terms of job opportunities, alumni network, etc.?
Yale and Harvard do not offer an undergraduate degree in business. Only two ivy league schools (Cornell and UPenn) offer an undergraduate business degree. See: http://www.quora.com/Why-do-most-Ivy-League-schools-only-Cornell-and-Wharton-do-and-Stanford-not-have-an-undergraduate-business-major
@gibby so if you want to study business you just major in Econ?
^^^ The majority of U.S. college graduates do NOT ultimately go into jobs specifically related to their concentration (or major). For example, my daughter graduated Harvard with a degree in film studies, but is going to medical school. And my son graduated Yale with a psychology degree, but is employed as a CS Data Scientist. Many of my son’s friends at Yale who majored in Math, Computer Science, Biology, English – even History have gone into financial consulting and banking.
With that in mind, what would majoring in Econ get you? What job, upon graduation, do you ideally see yourself having if you majored in Econ? What does “business” mean to you?
Just like law school, you can study anything undergrad and go to the top business schools in the country. Do well, get great job experience and network like heck.
If you want to actually study business concepts in college (accounting, entrepreneurship, etc.), Wharton is definitely your best choice, since (as the others have pointed out) Yale and Harvard don’t have undergrad business schools. You can take a few classes at Yale’s School of Management while you’re an undergrad, though.
If you’re asking about whether you can get into business directly after college, then Yale is basically as good a place for that as Harvard and Wharton. My sense (just based on what I’ve seen on the Yale, Penn, and Harvard online recruiting portals) is that the most business employers recruit at Penn/Wharton, and specifically, it seems that more finance and consulting firms recruit at Harvard and Wharton than at Yale. However, if you’re proactive and take initiative, you’ll do more than splendidly at Yale.
In the long run, the school isn’t going to matter. Good luck! 
If you want to study business, Wharton is the best place for that, no doubt. It has better on-campus recruiting (Wharton is preferred to Yale by investment banks, hedge funds, venture capital, etc.). Wharton also has a better alumni network in business, especially in finance. It teaches you many of the practical skills you’ll need to get a job at more competitive firms such as hedge funds. While its name is not as recognized on the street as Yale’s, its reputation in the business world can only be matched by Harvard. That said, Yale is by no means a weak school when it comes to business. It just doesn’t specialize in the field the way Wharton does.