From what I've seen, becoming a PM does indeed require serious breadth of smarts. I don't know about MSFT, but at Google being a PM requires technical skills, communication skills, design skills, and market insight. I interviewed for a associate product manager internship at Google, which is extremely selective (I was told they only take 10-15 APM interns a year). I had two on campus interviews with PMs, then an interview with a PM at Google (most people have a technical interview at this point too, with a Google engineer, but I had interviewed for engineering in parallel so I had effectively had that one already), and then the final step was to write a three page essay to be reviewed by a hiring committee. I didn't get the internship, but they had told me that the essay was the final round either way. Four interviews and an essay might not sound like a lot, but keep in mind this was just for an internship.

The APM interviews and the essay were very nontechnical. They mostly focused on product design and consumer experience (not limited to web/tech stuff), as well as one or two about industry in general (which I was pretty crap at answering). The most technical APM interview question I had involved coming up with strategies for improving AdSense performance and helping suggest keywords to advertisers. However (if the technical interview that regular APM candidates get were like the ones I got as first-round engineering interviews), the technical interview is pretty tough. Not impossible, but harder than MSFT or Amazon first-round...probably on par with MSFT second-round. And as a CS student I found the APM interviews the hardest of all.