Put the potatoes in a large pot and add salted water to cover. Bring to a boil over medium heat and cook until tender when pierced with a knife, about 20 to 30 minutes. Drain the potatoes and let cool slightly.
Meanwhile, preheat a grill or grill pan to medium-high heat. Add the corn to the grill and cook, turning occasionally, until evenly browned on all sides, about 10 minutes.
Put the potatoes in a large serving bowl. Using a sharp knife, carefully cut the kernels off the cobs and add them to the bowl. Add the poblano chiles, scallions, sour cream, mayonnaise, and cilantro, and fold together. Season the potato salad, to taste, with salt and pepper and serve.
To char the chiles (or any fresh chile): Put the chiles over a gas flame or underneath the broiler and cook until they are blackened on all sides. Enclose them in a plastic bag and let stand for 10 minutes to steam, which will make them easier to peel. Peel, stem, and seed the chiles. Once peeled, the chiles may be chopped, sliced or stuffed.