We know that Harry survived Voldemort's killing curse because Lily loved him so much that she sacrificed herself in an attempt to protect him. This resulted in a very powerful charm that pretty much rendered Harry immune to Voldemort (until Voldemort figured out the work-around).
Then there's this passage describing that night:
[Voldemort] was over the threshold as James came sprinting down the hall. It was easy, too easy, he had not even picked up his wand ...
"Lily, take Harry and go! It's him! Go! Run! I'll hold him off -"
Hold him off, without a wand in his hand! ... He laughed before casting the curse ...
Harry Potter and the Deathly Hallows Chapter 17: Bathilda's Secret
So James Potter, fully aware that he was sacrificing himself (he said "I'll hold him off," not "I'll get rid of him") ran to face Voldemort because of his love for Lily and Harry. This seems to be very similar to how Lily tried to protect Harry from Voldemort even though she knew she would die. It seems to me that the same charm should have been cast upon Lily when James died, but just moments later Voldemort killed her too. Apparently James' sacrifice did not result in a charm that protected Lily, even though he did it from love for her and Harry.
Why is this? Was something different in the circumstances?