Apple likes to have multiple suppliers for their components. That way there is no single point of failure, and Apple can start bidding wars between suppliers. As in "Brand X can make these parts for 52.3 cents each in lots of 1 million. If you can't match that, we'll take our business elsewhere..." So the TSMC deal makes perfect sense.
There is, of course, the small matter of legal action against Samsung. Which may or may not affect Apple and Samsung's business relationship.
So what about Intel? Apple supposedly served notice to Intel, warning them that if they didn't reduce the power consumption of their mobile x86 SoCs that they'd go elsewhere. Intel's response was to spew $300 million into the Wintel community in an attempt to bribe them into copying the MacBook Air. Irrational, short-sighted, and reeking of desperation. Much better to spend the $300 actually improving their own product and/or process.
Then what will Apple do if Intel ignores their demands? Who could they go to? Well, there's always AMD. They are the second largest maker of x86-compatible chips and they bought ATI in 2006. And guess what. Their market cap is only $4.6 billion. Apple could either acquire a majority of their stock or buy them outright. Then Apple could use their engineering talent to massage the AMD chips for lower power consumption.
Apple acquiring AMD is a long shot. Especially considering that it is also possible that Apple could be planning to migrate some or all of their Mac lines to ARM-based chips. That would take years, considering how long it took for Adobe to migrate their professional suites from OS 9 to OS X and Cocoa. But I'm sure Apple could sell millions of MacBook Airs even if they don't run Adobe bloatware.