Starbucks (SBUX) employees driving to the corporate headquarters in Seattle are greeted with a billboard proclaiming "4 bucks is dumb." It's part of the campaign for McDonalds' (MCD) latest offering: Espresso - and the hamburger chain plans to slug it out with Starbucks on price. That looks like a smart move in the current economy. Other billboards in western Washington State including some near Starbucks declare "large is the new grande" as McDonalds launches class warfare against what it sees as a hopelessly snobby competitor. So far Starbucks has remained blissfully above the fray. The bet: Its customers would never ...