With Google buying the company formed by Apple's just-departed PA Semi engineers, the rumored Apple purchase of ARM might be a defensive measure intended to make sure that it didn't get frozen out of the best designs in the ARM segment. Google's purpose might be to provide set-top web access on every television it can reach, which would lead it into competition with Apple's "still-just-a-hobby" project, AppleTV.
Another purpose is margins. Apple could foresee a future in which companies are forced to compete on price for functionally similar devices (especially as patents expire), and Apple has announced it wants to own the primary technology in every device it creates. The statement, first given by Jobs years ago, was echoed by Tim Cook more recently. By owning the ARM technology, it will be the only ARM vendor with no per-unit ARM licensing fee. This is a margins win. It also allows Apple to contribute to and guide future ARM development, know about the ARM roadmap, and attract engineers with a desire to have their work in devices all over the world. Apple could license the things ARM now licenses, and collect royalties, without sacrificing the ability to deliver unique implementations.
Since the value in ARM is in its worldwide licensing, it might be hard to see Apple demolishing that overnight. Just as Apple maintained PA Semi's military supply contracts when it bought the company, Apple will surely maintain existing licenses even if only to avoid the expense of buying them out of whatever contract term exists in the ARM license.
And I wonder what goodies Apple Legal might consider inserting in the next ARM license. Hmm.