From the Star-K's website:
Due to the flood of consumer inquiries regarding what can and can not be purchased at local coffee shops such as Starbucks, the Star-K has compiled the following information:
* All unflavored, roasted coffees (both regular and decaf.), may be purchased in a disposable cup. Sugar may be added. Milk (not creamer) can be added, and is cholov stam.
* Creamers and flavors may be added separately after the consumer verifies that the label of the original container bears reliable certification.
* Only packaged food items bearing reliable certification may be purchased.
* At Starbucks, bottled beverages bearing a KD are certified kosher, dairy, chalav stam, by R’ Zevulun Charlop.
* Frapuccino, whipped toppings, and other beverages prepared in coffee shops are not recommended since they are made in carafes/pump pots that are not exclusively used for kosher beverages.