From gamespot forums:
"Undead(Including heros) regen only at night(At double the normal rate). Undead and Demons gain combat boosts at night, and in the rain."
Is that what you were looking for?

Thanks captfitz, that's exactly the sort of thing... but are there others? I read somewhere - no longer sure where - that Fey are disadvantaged during storms (or maybe have bonuses during fair weather, regardless of day or night). It seems reasonable that the Ssrathi might have bonuses in the day - their temple casts a spell to change it to day - but I don't know if there's any real effect.

I found a bit more information for my own question, from the patch notes:
At daytime HE gets +2 combat, on fine weather WE and Fey get +2, Undead don't benefit from rain