I found a great website that lists the vegan options at Starbucks, check it out here: http://www.veggywood.com/2010/05/07/finally-starbucks-ingredients-list/
Basically, you have the option of every regular syrup with your soy lattes. The surprising options are: mocha syrup, macha powder, vanilla powder and strawberry syrup that can make soy strawberry/vanilla/green tea frapps!
The only advice I give is to watch them make the drinks to make sure they are actually using the soy! We've caught the baristas forget to use soy and try to give us a regular milk latte before. For hot drinks they will pull out the soy milk carton and use a smaller tin container with "SOY" on the side. The cold drinks will be put in a yellow blender container.
Sadly, the caramel frapps are off-limits but the caramel pumped syrup (not the golden gooey stuff) has the same flavor and is vegan.