Whether you think this is simply a PR spin or a way for an agency to regain confidence after losing a major client, I think that the act of resigning a client is something that shows the backbone of what an agency stands for.
Starbucks and W+K have been together for a long time. They've been through the good times (the launch of the brand, the rise in sales and store openings) and now the bad (the decline in the American economy resulting in hundreds of store closures, the lack of client focus - have you seen all the new CD releases at the counter? - and the general panic that comes to marketers when their brand is losing strength).
Although the full details will never be released, my guess is that things weren't just getting tough, they were getting downright awful. Agency people left for better things, the marketers might have considered briefing multiple groups on the same project, executions were getting criticized because they weren't driving short-term objectives...who knows.
But the decision for W+K to leave them is a bold one. It says that the culture of an agency is more important than a client. That the quality of the work is more vital than hitting the revenue number of the month.
It takes guts to do that - to have the confidence that your shop will continue to thrive, that it will find new business and that it will make up for the lost revenue over the last year.
It would have been easy for W+K to pander to their client. To spend "a bit more time" hoping that things got better. But sometimes you know the decision that has to be made. And usually, it's only the client who has the guts to pull the trigger.