It wouldn't be overnight, but Apple's business would no longer be sustainable. Samsung and a dozen Chinese companies would instantly start shipping iOS devices. Sure, the hardware wouldn't be quite as good as Apple's, but they'd still steal a big chunk of Apple's market share. Then it'd be a few years of slow bleeding where they subsidize their competitors, unable to compete on price and eventually die.

This same dynamic would apply to Macs as well, of course. In fact, this very thing came this close to killing Apple in the 90s. They licensed MacOS to clone makers, who immediately took the high-end high-margin market, and left Apple to develop MacOS and sell low-margin, entry-level machines. It took the return of Steve Jobs to turn the company around. They were weeks away from bankrupcy. He immediately killed the clone makers, begged/threatened Microsoft into giving them bridge funding, and started work on the candy-colored iMac.

As for Android, well, it's nominally open source, but an awful lot of the stuff you need to ship a competitive OS is actually closed and closely guarded by Google. It's a bit more open than Darwin, but not much. If you stick to ASOP, you've basically got Froyo.

So, yeah, Chromium and is for real, and again it's a bit more open than WebKit in that it also includes the app chrome, and not just the web engine. But coincidentally, Google benefits from an open, standards-based web; it doesn't threaten their revenue at all. Go figure.