This is what goes through my head:
Apple buys ARM. Continues to make money off of licensing, keeping custom projects solely proprietary for Apple. Enables more complete control of product design from both hardware and software end.
Eventually, Apple incorporates ARM A4 derivatives into all of its devices. A new version of OSX (perhaps 11?) will be written to be platform native to ARM. This would allow essentially "full", unfettered OSXI to be running on all Apple devices. The only difference would be the "depth" of the user experience; those users wanting to to only iPad-like app stuff would boot up with the A4 only and enjoy ridiculously long battery life. Users wishing to do more heavy lifting or content-creation could boot up into a more OSX-looking version.
Of course eventually these devices would all be totally interoperable, as would all apps on them. Imagine having a version of "Pages" that you could purchase over the air through the App Store that, once installed, contained a version that looked and operated differently based on the product you use it with. For example, when using your iPad it would look iPad-ish. When you dock your iPad (maybe even inside a giant Apple monitor?) or connect it to your workstation iMac or Mac Pro, all the files immediately transfer and your Pages app can be used on the iMac etc in "full computer" mode.
Apple has all the tech and the framework already right there to do this. All they need is a massive server architecture (under construction) a robust MobileMe option to keep everything succinct (in progress, needs development), a portable netbook replacement thingy to push the envelope of portable device computing (iPad, in its infancy), and the design and technical capability to run different versions of its software on different chipsets and architectures without compromising usability. Buying ARM would enable more flowing software-hardware integration.
I can see the day when ARM stuff powers all Apple devices and works very well due to contiguous designing of both software and hardware for optimized performance. Imagine a Macbook with chained-together super powered A4 incarnations enabling something like a WEEK of battery life with the capability to boot into iPhone OS or OSX depending upon user wishes? In fact, go one step further and enable the iPad over time to achieve all the capabilities of the Macbook line.
It's going to happen. All the dominos are set up.