Abstract expressionism isn't a medium though, but a philosophy. Abstract expressionism could be expressed in pixel art if one so desired. It was important for the entire art world, because it switched the emphasis from product to process. It's a reaction against movements that predate it. I don't think pixel art is effecting the entire art world in any significant way, I don't think it's a philosophy, and I don't think it's a reaction against previous art movements (in itself, anyway).
So I don't see any reason why pixel art should sit alongside abstract expressionism or surrealism or any of the other movements. Because I think it's a medium and not a movement. There exists in the art world a sort of hierarchy of media, weaker than it once was perhaps but no less prevalent. Painting and sculpture are generally at the top, and the newer media (printmaking and photography, for example) have been catching up. It'd be nice to see pixel art rise in the list, so that it's no longer simply a novel or nostalgic mode of expression, but these things take time. Currently, as some have already noted, it's probably too tied to its practical application in videogames (and other digital media: forum smilies, etc.) to get its butt off the internet and into the gallery (or the art history books, anyway).