Mars is a forbidding place. Despite dreams of "terraforming" the planet into a future paradise, it is far from paradise now. Mars is bitterly cold (temperatures a hundred degrees below zero or more), is covered in a very alkaline dust (which tends to be corrosive), and it is subject to dust storms that envelop the entire planet.
A few weeks ago, one of those dust storms started. It began as a regional storm (like a frontal system on Earth), but has blossomed into a planet-encircling storm. If you look at the weather movies taken by spacecraft orbiting Mars, you can see huge clouds of dust moving across the face of Mars.
As both rovers are solar-powered, they need sunlight for electricity, and they need electricity to run not only science instruments, but heaters that keep the joints of the spacecraft from freezing in the Martian cold. The picture above shows the view of the horizon seen by Opportunity from its perch next to a crater on Mars. You can see how it has gotten much darker as the dust storms have progressed. As of a few days ago, only 0.5% of the sunlight normally hitting the surface was getting through unhindered; a larger amount was still reaching the ground. The situation is like a foggy morning -- there is light to see and light to power the solar panels, but it is much darker than normal and hard to see very far.
The Mars rovers may very well not survive. If the dust stays this thick, the batteries will slowly drain due to the needed heaters, and the rovers will fall silent. If it clears, both rovers may still be in perfect health. Opportunity is ready to journey into its biggest crater yet, where it may see millions of years worth of rock and water action from early Mars.
We don't know when the storms on Mars will subside. Sometimes these storms last only a few weeks, sometimes a month or more. And even then, it will take weeks for the dust to settle enough that plenty of sunlight is reaching the ground. You know how bad weather forecasts on the Earth can be; now imaging trying to forecast what is going to happen on another planet when we don't know the temperature, wind speeds, atmospheric pressure, or any of the numerous things that could be driving the Martian weather.
Even if the rovers do fall quiet, they have far exceeded their design goals and any expectations we had for them. And for that, we should be very happy.