While rival Starbucks pledged to offer a free cup of premium arabica coffee yesterday to customers between 10 a.m. and noon, Dunkin’ offered its own customers free cab rides in downtown Boston. Starbucks estimates to have poured more than 500,000 12-ounce cups. For customers who couldn’t make it to the restaurants, Starbucks employees took coffee to them using mobile-sampling “Venti Vans” and insulated coffee backpacks.