There are many conflicting and competing definitions of flat design. It's generally agreed that Apple will go flat, but nowhere near as flat as Win8 or even a lot of the web.
Don't worry about anything until you see the keynote. If you want to mesh with Apple's new visual design, you really can't do anything without seeing said visual design.
Seriously a well thought app is a well thought app, you don't need to go flat just because it will be the next 6 months crave before textures pops back again....
Skeuomorphic doesn't necessarily mean photorealistic. Flat doesn't necessarily mean white text on bold colors. I believe you can have a design that is both skeuomorphic and flat, though I don't expect them to do that.
I think it's safe to say Apple will move away from skeuomorphism, but that doesn't necessarily embrace some pure form of what we've been calling "flat" design.
Considering that many skeuomorphic designs involve a layout which spatially mimics some particular object. One can't scale that up and down in size to fit on any screen without also changing the size of the actionable interface areas (buttons, toggles, and such). And since the controls have their meaning implied/explained to the user by where they are on the layout of the mimicked object, rearranging the location of control areas takes away from the meaning that the mimicry is made to communicate. If the physical object mimicked is larger than the screen, and the layout and control area sizes must be preserved, then scrolling the entire view or something like it is needed.
With a flat interface, the actionable control areas can be grouped, for example by type (do they alter something global, domain related, context-specific, or only a single item). Membership of a control in a type could be communicated to the user by something like grouping all the controls of that type together in a box with a particular background color. Since the scope meaning of the controls is encoded by which color box they are each in, but not at all by where in that box they are, nor necessarily what shape that color box is, nor the box's location relative to any other color boxes which hold controls of different types, then the color/type boxes can be reshaped and reflowed to fit many screen sizes while keeping the size of the controls themselves constant (easy to hit with a finger, or mouse, or whatever).
Edit-addendum: Of course, this line of thinking simplifies skeuomorphic design too much. Such designs allow for more conceptual or cartoonish representations of the controls from familiar objects, so that the scaling factor used for any given control doesn't have to correspond to the scaling factor of other controls or the overall mimicked object. Still, flat design gets a great deal of rearrangement of controls for the price of the user learning to associate the box with color scheme foo as holding things that change the global context of the app. And the amount a user needs to learn can be smaller still. Even just using the principle of up-and-to-the-left is more general while down-and-to-the-right is more specific can give flat designs some structure (enough for many uses) while allowing a great deal of rescaling of the screen.
