Well, here's a distillation of what I've been seeing on various "Cloverfield" discussion pages, including reports from people who claim to have seen preview screenings. POSSIBLE SPOILER ALERT! (Select following text to make it visible.). The monster is roughly Godzilla-sized, and first appears in the mid-Atlantic, where it destroys a deep-sea drilling platform. It has six or more limbs and a "whale-like" head. It's underbelly is covered with large parasitic arthropods, which drop off once the host moves onto the island of Manhattan. While the host monster smashes the city, the arthropods reproduce by injecting wee arthropods into human hosts.
Sounds like a pretty good monster to me, in part because it sounds a bit like the creature in Mike Mignola's "Hellboy: The Conqueror Worm" comics. If the monster's initial appearance is as described, it certainly nods to The Big C, and could be considered Lovecraftian in much the same way that "Alien" was Lovecraftian.
I wasn't crazy about "Blair Witch Project," mostly because the actors were quite limited, but I did appreciate its story structure and economy of style. I hope "Cloverfield" lives up to its potential, and doesn't become a case-study of how a viral marketing campaign can surpass in achievement what it tries to sell.