Preheat the oven to 400 degrees F. In a small skillet, toast the pine nuts over medium heat until golden brown, 2 to 3 minutes; set aside.
Trim the top and bottom of the fennel bulbs. Remove the first outer layer, and cut each bulb into 6 wedges. In a large bowl, toss the fennel with 3 tablespoons of the olive oil, the vinegar, salt and some pepper. Place it in a single layer on a rimmed baking sheet. Roast until golden brown around the edges, about 45 minutes, making sure to flip the fennel over halfway through. Allow to cool to room temperature.
Add the roasted fennel to a large platter, and scatter the orange segments and the arugula over it. Give everything a light toss, but be careful not to bruise the arugula too much. Drizzle the remaining 2 tablespoons olive oil and a touch more balsamic vinegar over the vegetables. Season with salt and pepper, and sprinkle the toasted pine nuts over the top.