I found this AMAZING product, Manhattan Special Diet Espresso Soda. I don't know if it's available outside of the NYC metro, but you can order it online. 1/2 cup, with 1/2 cup milk tastes like the most decadent egg cream you can imagine. Even my totally diet-adverse BF is hooked.
When I can't locate it, which is pretty often, I go with Waist Watchers Chocolate Fudge Soda and milk. Not as rich, but still a great P1 treat.