First: the contract! That's right, JG is signed on for (I believe) at least 2 sequels. Now, if her character is too busy chilling 6 feet under, how on earth is JG going to fulfill those sequels? Yeah, yeah, I know the possiblities- flashbacks! Ghost! Dreams! Alternate universe! And you know what I say to that? WHAT-FUCKING-EVER! Would YOU want to commit yourself to 2 movies, which would each take 4 or more months to film, if you were just an apparition of some sort? (Yeah, I probably would, but I'm me, so shut up.) It doesn't make sense. Jennifer Garner is turning into such a huge star, between ALIAS and now Daredevil, that she was OFFERED a role by Steven Spielberg! Really, with those kind of bragging rights, one doesn't just hang out and play dead- literally.
Second: The necklace. If you saw the movie (and if you didn't why the hell are you spoiling yourself? Spoilers are evil, didn't you read my rant, you fascie?) then you know what I'm talking about: Matt Murdock asks Elektra about her necklace, she explains, and he semi-jokingly wonders if he can get one in braille. Now, at the end of the movie, after she has supposedly bit the dust, Matt wanders the city and finds a necklace just like Elektra's, in braille, hanging somewhere. Coincidence? I THINK NOT! Honestly, how many people go around and hang necklaces on New York City rooftops? That's right.
Third: The comics. If you've read any of the comics (or at least did research on the whole backstory, as I did in preparation of the movie) you'll know that Elektra comes back to life (yes, she died, but she's back, so there). Hell, she visits her own grave! There's this quote from the Elektra series that I really like: 'My name is Elektra Natchios...this blade I am holding- I have died at the end of it...I tell you this because I want you to know that every time I use this blade... every time I slice into a man's flesh with it... I know exactly how it feels.'
Fourth: Jennifer Garner herself said so! That's right. Recently on the Jimmy Kimmel Show, JG was a guest. Kimmel asked her about the end of the movie and Elektra's death. After seeming to get a bit annoyed about him giving away the end of the movie, JG said that Elektra's not dead. She pointed out exactly what I did (the necklace and the comics) and assured him that Elektra would be back. Kimmel's response? 'What the hell is she doing making necklaces?' Yeah, he's intelligent.
And last, but not least: the proof is in the pictures! Or at least it would be if I hadn't accidentally deleted the picture... but that's ok. The rant itself shall suffice.