Hopefully all races won't wear any underwear.
And be horrible, terrifying monstrocities with what would lie under the underwear being completely smooth? No thanks.
Bethesda is NOT going to do nudity, I say that with 90% certainty, why 90%? Because I find it highly unlikely that they would do it, but I don't want to say it's 100% certain since my assumption is merely based on what past experiences have led me to believe Bethesda's current stance on the matter is, not on anything that has been said about Skyrim, and if we can't have full nudity, having characters be nvde without having actual nudity is just unnatural and unrealistic, and it only looks stranger as the level of realism of the rest of the characters' model increases, if you don't want to show nipbles, then cover them up, that's a much better approach than having characters be nvde without any details visible.
As to the styles of underwear, we don't really know, though if we assume that different races have different body models again, it would not be impossible for them to have different underwear styles as well, though it's by no means guarenteed, after all, in Morrowind, all races had the same ugly briefs, except Khajiit and Argonians, only because they didn't have any. In any case, it's not something I have a strong opinion on, since I don't expect to run around naked much in the game, as long as the underwear is there (I have nothing against full nudity actually, but like I said, I don't think we'll see that, so I'd much rather have permanent underwear than no underwear but nothing under it either.) and the underwear isn't absolutely hideous, I don't really feel that the way my character's underwear looks is vital to making my character feel unique, nor will I feel that the different races aren't clearly defined enough if they don't wear different styles of underwear. Though no matter how it looks, there's a good chance I'll eventually get rid of it or replace it with something else using body mods anyway.