As far as I know, Apple made the glossy design popular with the iPhone icons. But in recent years, seems like app designers/devs started to embrace the flat icons.

Some famous apps like Flipboard:

Evernote:

And even Monoco, the app I'm working on!

I noticed this trend started with Microsoft's not-to-be-called-Metro style for Windows 8 (or RT?). Maybe it started even before, but it if this is true, Microsoft has done something right this time.