1. Cook pasta according to package directions, adding the corn the last 5 minutes of cooking. Drain pasta and corn. Rinse with cold water; drain again.
2. Meanwhile, for dressing, in a small mixing bowl stir together sour cream, salsa, cilantro, and lime juice. Set dressing aside.
3. In a large bowl combine pasta mixture, black beans, tomatoes, zucchini, and cheese. Pour dressing over pasta mixture. Toss lightly to coat. Serve immediately or, if desired, cover and chill.
Prepare salad; cover and chill for up to 24 hours. To serve, if necessary, stir in enough milk to make desired consistency.