Now that the dust has settled over the announcement last week that Microsoft had licensed ARM's architecture for undisclosed plans, it is now time to consider what might have pushed Microsoft to do such a move and most importantly, why the firm, unlike Apple, announced it publicly.
(1) Control over design
One week before Microsoft announced its partnership with ARM, Steve Jobs, the CEO of Apple, said at the event where he announced free cases for iPhone 4 owners, that the company wouldn't get into any business where they did not own or control the primary tech because he said "if you don't the people who do own it will beat you". Looking at Microsoft's business model, the company, like Apple, owns only half of the technology. The other part being the property of Intel. So essentially, Microsoft is emulating Apple to reduce its dependency on Intel.
(2) Ability to alter design more efficiently
ARM and a few other tech majors have set up a joint venture called Linaro (more here) which aims at improving the Linux Kernel. Microsoft may be aiming to achieve the same thing by licensing ARM's technology. By tweaking Windows Phone 7 for ARM and potentially designing its own chips (like Apple did with the A4), Microsoft could come up with some pretty unique selling points.
(3) Not put all eggs in the same basket
Intel has decided to gradually moving away from Microsoft as a partner and last year unveiled a new strategic partnership with Nokia, merging both Moblin and Mameo into Meego, a Linux based platform aimed primarily at mobile devices. Microsoft's move with ARM may well be a tit-for-tat attempt to reduce its dependency on the x86 architecture which was great for desktop and tethered computing but might just be too bloated for anything else.
(4) The rise of Mobility & Cloud computing
Amit Singhal, the guy pretty much in charge of search at Google, stressed the importance of the mobile segment and cloud computing. Microsoft knows that desktop computing may well be living its last decade as mobile devices become as powerful as (if not more) than existing desktop computers while being more polyvalent, cheaper to run and cheaper altogether.