If the comet ISON makes it out intact from its encounter with the Sun this week, the Earth is in for a potentially dazzling sight in the December night skies as it passes close to Earth and out further into the solar system. But scientists are starting to emphasize the "if" in that sentence a bit more today, after some observed what appears to be a sudden drop in the light emitted from ISON, along with an apparent explosion of dust production. Those observations could mean that the comet broke apart during its approach to the sun.
This, however, does not mean that ISON is definitely gone. Comets are basically masses of dust, gravel, and rocks, all bound together by ice. ISON is at the beginning of what could be its first and only pass around the Sun, and its orbit takes it extremely close to the star. This also makes it really hard to observe what's happening to ISON right now. As Slate's Phil Plait explains:
That’s not the only possible explanation! It's been suggested that ice is distributed approximately evenly across the surface of the comet, and is finally starting to be depleted. The comet may yet still be intact, and this observation was so recent (from earlier today) that we don’t yet know what’s going on.
The NASA ISON Comet Observing Campaign adds, "we do still need to keep observing the comet to be sure what it happening...the last time we saw an object like this was never!" So basically, even as far as comets go, ISON is pretty darn hard to read, although the observing campaign says today's observations are a "little worrying." It's way too early to say whether ISON is still with us.