This post has spoilers. Duh.
Before I saw the movie, it seemed fairly similar, from previews and whatnot. After seeing it, I'm much less convinced.
There were a lot of similarities that are trivial, things from mythology (sunlight kills vamps, silver hurts werewolves, get bit to get infected, blah blah). There were also some differences from mythology, but they aren't WW-specific. For instance, Underworld vampires didn't have to be invited into a house. WW vamps don't either, Buffy vamps do, Anne Rice vamps don't, Blade vamps don't. Flip a coin, apparently.
Underworld vamps and werewolves were infected beings, you get the virus and you turn. You don't have to die first to be a vamp, apparently. That's not WW at all.
Underworld werewolves weren't clan-based, there was just one group of them apparently. They were also apparently immortal, like vamps. Not WW.
Underworld vamps weren't clan-based. There was apparently an american coven and european coven, but that's not WW-specific, anne rice and Buffy both had organizations of vampires. Had there been different clans with different powers, I might have bought it.
Never really saw many powers from either group, except they were all strong and fast. Eh. The vamps seemed to get stronger as they got older, but that's not WW specific either. There was no concept of generations like you find it Vampire: the Masquerade.
The part that killed most of the similarities for me was the virus. Both groups were descendants of one man, some nobleman in the 4th or 5th century, who survived some kind of plague. His children inherited the virus, which made one of them a vampire, one a werewolf, and one normal but a carrier. There's nothing like that in WW at all. WW is "Caine is the angsty forefather of vamps" and "Garou are the tree-hugging children of Gaia", and they have no familial tie.
WW claimed in their suit that a bunch of trivial elements added up to make the Underworld setting similar enough to the World of Darkness that it was infringing. I didn't see it. The clans and tribes of the WoD are central to the setting, and they just weren't in the movie. The look and feel was "vampire-werewolf shoot-em-up w/Matrix", not WoD.
The one part that screamed WoD to me was the "abomination". The concept of it could go either way, and the method of creating one wasn't the same as the game world, but to use that name for that concept was pretty blatent.
The story WW is saying is similar, "Love of Monsters", I haven't read, there could be some plot similarities in there. That might be where they got the "abomination" concept too. So the whole case really has to do with that story, I think, because the movie isn't very WW-ish at all.
They set it up so there could be a sequel, and this one was fun eye-candy for a couple hours, so I'll probably give the sequel a go too.