I don't think of flat as being new at all. The cave paintings of Lascaux were flat. The art of the Egyptians was flat. The geometric shapes of Islamic art were flat. Bauhaus was flat.
Software design traditionally employed skeuomorphism as a way to help users adapt to interactions on a flat display: real world attributes like depth, texture, and lighting effects were thought to aid affordance.
Now, though, users are comfortable with a flat-interaction paradigm; they no longer need skeumorphic cues to guide them through most UIs. The information presented on-screen can now be as simple and minimal as the devices on which they are displayed.
So, is flatness a trend? Sure. It's a way to signal that software is new, sleek, modern, less clumsy and cluttered than the past. Does this mean depth won't make a comeback? No. Design is cyclical. However, I don't think depth will ever come back because it's needed, because it helps users unfamiliar with a digital world navigate as though it's a physical one. It will come back as a way to differentiate one product from another.