Bring a large saucepan of salted water to a boil. Add the 6 cups of arugula and blanch for 10 seconds. With a slotted spoon, transfer the arugula to a colander. Rinse under cold water to stop the cooking, then drain.
Add the couscous to the boiling water and cook over moderately high heat, stirring occasionally, until al dente, about 10 minutes. Drain the couscous and spread it out on a large baking sheet. Drizzle lightly with olive oil and toss to prevent clumping. Let the couscous cool to room temperature.
In a small skillet, toast the pine nuts over moderate heat, tossing, until golden brown, about 2 minutes. Let cool.
Squeeze the excess water from the arugula and coarsely chop it. Transfer the arugula to a food processor. Add the pine nuts, garlic, cheese and the 1/2 cup of olive oil and process until the pine nuts are finely chopped. Season the pesto with salt and pepper.
Transfer the couscous to a large serving bowl and stir in the pesto. Gently fold in the tomatoes. Garnish with the arugula leaves and serve.