If it's anything like the vanilla effect, it probably just does the "decreased stats" and "no regeneration" stuff, which isn't too exciting. I don't have Dawnguard yet, though, so I have no way of checking at the moment.
Oh yes, I think there are new spells in Dawnguard that have something to do with the sun or sun damage or something like that. I'm not certain how this works, though.
Don't know if this helps any but if you look under weather and check SkyrimClear Vs SoulCairnAmb01 there is a sun damage slider with it set to 1 Vs 0 respectively.