In a shallow dish, mix together olive oil, cumin, chopped cilantro, and lime juice. Season with salt and pepper. Add chicken, and mix well. Cover, and refrigerate for at least one hour.
Preheat grill for high heat.
Thread chicken, zucchini, onion, red bell pepper, and tomatoes onto skewers.
Brush grill with oil, and arrange skewers on hot grate. Cook for approximately 10 minutes, or until chicken is cooked through, turning to cook evenly.