I know this isn't very helpful on a technical level, but...
The new iPad SDK site specifically mentions the term "Universal Applications" with the following:
Developers can now start planning for
universal applications, allowing them
to take full advantage of the
technologies found on iPad, iPhone,
and iPod touch with a single binary.
I know this doesn't explain ''how'' to take advantage of this idea, but I'm not quite willing to drop 100 dollars to learn more just yet.
But this does resolve your core goal, which is designing apps that can work nativly on both devices. With the a universal binary, you wouldn't need to worry about covering idiosyncrasies for two versions of one project unless you wanted device-specific features.