The Huffington Post has an interview with Guillermo del Toro, in which the director reveals he and Charlie were thinking about adapting Vonnegut's Slaughterhouse Five. It's near the end of the article:

You know, the one I'm most disappointed that it looks like it won't happen is your version of "Slaughterhouse-Five."
I will tell you, my idea was to get Charlie Kaufman to write it. And I spoke to Charlie Kaufman about it and we came up with an idea on how to approach it, which I thought was very, very interesting. But, it was right at the time I went and started "Pacific Rim." So the studio, they didn't want to invest in that project if it was not going to be my next movie. So, you know, it gradually cooled a little bit. But the exact take I proposed to Charlie Kaufman is the exact take I would do with the material.

I have to admit, I would have loved to have seen that combination with that material. It doesn't seem like an easy book to adapt.
No, but, then again, Charlie Kaufman is the guy doing it. How can I put it? The first "Slaughterhouse-Five" movie that was done was a really good movie, but it's about flashbacks and flash-forwards. And what is gorgeous about the book is that he becomes detached from time.

Right, time is all happening at a once.
The Tralfamadoians say, "Like we can see a mountain range. We can see the alive, we can see the dead, we can see ourselves at age five and we can see ourselves ancient. It makes no difference." It makes no difference. And that was the idea that we were talking about. We were talking about how it was going to be very experimental. But, you know, if I had the money to pay for any of these movies, I would do it. I would do it in a second.

So is that 100 percent dead?
No, no, no. It may still happen. But, I don't control the material. Let me put it this way: when I control the material, I never give up on a movie. I mean, "The Count of Monte Cristo" was 15 years. "Devil's Backbone" took me a decade or more. But, if I don't control the material, I have no say. It's a property of Universal. (Source)

I don't know if any film could do justice to the book (on the other hand, movies are not books and shouldn't really be compared to each other, eh), but this would've been really interesting, no?

Thanks to Nathaniel!