If you've been using IB and setting the resize behaviors of elements properly, and also coding frame coordinates all relative to each other you are half-way to having a UI that can potentially scale to a larger screen.
From the screen shots there are new kinds of action-sheets as well, potentially attached to UI elements instead of floating - if you use overlays today they will probably work about the same but you may want to consider changing placement from the center on larger display.
UPDATE:
Now the event is over, and registered developers can download the SDK - although we cannot talk about specific features here just yet, read through ALL of the documents related to the new OS version as there are a number of things aimed at helping you transition to supporting both platforms. Also before you start using custom libraries for things take a look through the API changes to see what new abilities might be supported that are not today.
Generally speaking, what I said above about IB holds true, and also you should start thinking about how your apps today could use more space to present more information at once instead of being split out over multiple screens. Also if you are doing any projects right now that use images, make sure to initially design the images large enough that you can also use them for higher resolution tablet applications.