Put the potatoes in a large pot and cover with salted water. Bring to a boil, lower to a simmer and cook until tender when pierced with a knife, about 30 minutes. Drain the potatoes and let cool slightly.
Meanwhile, prepare a grill to medium-high heat. Add the corn and grill, turning onto all sides, until evenly browned, about 10 minutes.
Halve the potatoes and put them in a large bowl. Using a sharp knife, carefully cut the corn kernels off of the cobs and add to the bowl. Add the poblano chiles, scallions, sour cream, mayonnaise and cilantro and mix gently to combine. Season with salt and pepper and serve. (The potato salad can be made 1 day ahead and refrigerated. Let stand at room temperature 30 minutes before serving.)
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.