Cook pasta and tomatoes in a large pot of boiling salted water until al dente, according to package instructions. Reserve 1/2 cup pasta water. Drain pasta mixture, and return to pot.
Add arugula and butter to pasta. Toss, adding reserved pasta water a little at a time to create a thin sauce that coats tortellini (you may not need all the water). Season with salt and pepper; toss to combine. Serve.