Hello all,
It is finally time for me to join the discussion, now that I have the time and perhaps some expertise that may be helpful. This still could be a cave. In fact, I was encouraged to see the picture as it confirmed an idea that I had when I first read about the cave. I have been a caver for many years, and now I can put that experience to use.
There exist three types of cave - at least on earth:
1. Solution caves, formed by water disolving away limestone, which is itself a sedimentary rock formed with the help of water. Given the amount of water on Mars, I would have been very surprised to see a cave of this type.
2. Talis or boulder caves, formed when rocks and boulders pile up during and after a landslide. This might have been a possibility, but they can be very dangerous, as they are often filled with loose boulders.
3. Lava tube caves. These caves are formed near volcanos, where liquid lava was moving. A stream of lava will often harden at the top and sides, leaving the lava to run through a tube in the center of the flow as it cools. When the source of lava dries up, or cools off, the remaining lava in the tube runs out leaving a lava tube cave. Lave tube caves have a shape that is very easy to recognize, having a circular or oval cross-section. They are also usually relatively flat and easy to explore. The chance of finding a 50' or 100' pit, as is common in many solution caves, is very low in a lava tube cave. Most lava tubes are also fairly short: a few hundred feet is a typical length. Lava tubes more than a mile long are very rare, and there are seldom any brancing passages, either.
As this cave is supposed to be near Olympus Mons, I figure that it must be a lava tube, if it is naturally produced.