In a small bowl, whisk vinegar, shallots, mustard, and salt until thick and emulsified. Gradually whisk in olive oil.
In a 3- to 4-quart pan over high heat, bring 2 quarts of water to a boil. Add corn kernels and cook for 1 minute; drain, rinse with cold water until cool, and drain again. In a large bowl, combine corn, radishes, and arugula.
In a mortar, lightly crush coriander seeds (or place seeds in a zip-lock plastic bag and crush with a mallet or rolling pin).
Pour dressing over the arugula mixture and gently mix to coat. Sprinkle salad with crushed coriander seeds.