To prepare soup: Toast cumin in a small skillet over low heat, stirring, until just fragrant, 1 to 2 minutes.
Combine tomatoes, onion, 2 tablespoons cilantro and chipotle in a blender or food processor. Puree until smooth. Transfer to a large bowl. Add the toasted cumin, corn, ice water, lime juice and salt; stir to combine. Refrigerate until chilled, about 1 hour or until ready to serve.
To prepare cilantro yogurt: Puree yogurt and the remaining 1/4 cup cilantro in a blender or food processor until smooth. Refrigerate until ready to serve (it will thicken slightly as it stands).
To serve, divide the soup among 4 bowls and garnish each with a generous swirl of cilantro yogurt.
Canned chipotles packed in adobo sauce can be found in the Mexican section of a large supermarket or at a specialty food store.