Some of the most obvious body doubles ever on film...
I realize directors are in a no-win situation since you can't show the actor's face in the same shot, but the least you can do is use better angles, and definitely keep the body doubles' faces out of the shot!
After Christina emerges from skinny dipping in the lake, you can clearly see the body double's face through the hanging tree leaves -- very obvious it's not Kate Beckinsale. Also, during the underwater dive, the nude Christina's face is of different structure than Beckinsale's (though less obvious than the emergence scene).
When David looks through the window as Christina poses for the painting, the shooting/editing is very blatant. A wide shot shows a nude Christina looking away from the camera (thus hiding her face), cutting to an extreme close up of Kate Beckinsale's face. The bad shooting gives this one away.
Don't even get me started on the sex scene. You can clearly see the face of the David double for 4-5 seconds when Christina is riding him! The Christina double's face can also be seen from the side during these shots.
I've read through the threads and some of you don't think doubles were used. Seriously, pay closer attention. They're more flagrant in this movie than pretty much any movie I've ever seen.