Problem with ARM would be loss of compatibility with most existing OS/X applications.
Perfect (as far as Apple is concerned).
They'd LOVE for this platform to be brand new, the same as the iPhone was. So they'll have 3 types of app: iPhone/iPod - Netbook - and Mac.
Why? Well because using multitouch on programs designed for a mouse is awkward at best. How do you fix that? By making the developers do the work of re-doing their programs, of course! After the iPhone's success, I'm sure most developers will rush towards re-working their apps for a netbook or tablet. Let THEM do the work that needs to be done to make a multi-touch device the best it can be.
This has the added benefit that they'll probably cut down on un-needed parts of their programs. Will Photoshop CS4 run well on this thing? Doubt it. Will Photoshop, Netbook-Eddition work well after they trim out the fat (things that wouldn't be useful on a tiny screen anyway)? You betcha!
Forcing developers into doing that would be exactly what Apple wants.
EDIT: I bet we'll see the hardware announed at the developer's confrence, but shipping in 4 months. And, of course, some wonderful Apple-way of "easily" turning Mac apps into Netbook-apps that they'll be showing developers.
EDIT 2: I wonder if this will be the start of a "Finder Store." Exactly like the iTunes store, but runs in Finder instead of iTunes. It would be for Netbook and Mac software. Since these devices don't sync with iTunes it makes little sense for their software to live there. But the "Finder Store" could work exactly the same, and download apps to the Applications folder.
EDIT 3: It's probably obvious from what I've said, but I believe an Apple netbook will be more "Mac-Like" than the iPhone is, but it will be closer to the iPhone than to OS X in terms of the way it looks and works. Yes, there will be a Finder, and more customization, but don't think you'll get Mac OS X on this thing. It will most certainly run its own "Netbook OS X." I'd actually expect that it won't have the word "Mac" anywhere near it. It will be the "Apple Something-or-other."