On the Unmannedspaceflight.com and Uplink.space.com forums was discussion on small depressions in Meridiani that do not have the appearance of impact craters. One possible explanation that was discussed was that they could be manifestations of karst terrain:

Tiny Craters.
http://www.unmannedspaceflight.com/index.php?showtopic=943&hl=karst&st=195

The baffling case of the tiny craters on the Meridiani dunes.
http://uplink.space.com/showflat.php?Cat=&Board=sciastro&Number=375988

Karst terrain is due to acidic water dissolution of evaporitic deposits such as carbonate and sulfate:

A karst PRIMER.
http://csweb.winona.edu/semnwrb/files/general/sprkarst/sprkarst.htm

Here's a report that examines this possibility at Meridiani:

Possibility of Karst Morphology on the Martian Surface at the Meridiani Landing Site from Comparison with Terrestrial Analogs.
Sz. Bérczi, Eötvös University, Department of General Physics, Cosmic Materials Space Research Group, H-1117 Budapest, Pázmány Péter s. 1/a.Hungary,(****@ludens.elte.hu)
Lunar and Planetary Science XXXVI (2005) 1051.pdf
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1051.pdf

It gives this image as an example:

The report has a nice diagram showing that on Earth such karst terrain is connected to cave systems, which raises the possibility it might also exist at Meridiani.

This report discusses detecting cave systems remotely:

REMOTELY SENSED CAVE DETECTION ON EARTH AND MARS.
C. A. Drost, J. J. Wynne, M. G. Chapman, J. S. Kargel, T. N. Titus, and R. S. Toomey (US Geological Survey, 2255 N. Gemini Drive, Flagstaff, AZ 86001, ****@usgs.gov; Dept. of Hydrology, U. of A., Tucson, AZ 85721; Western Kentucky University, Bowling Green, KY 42101-1066.
Lunar and Planetary Science XXXVII (2006) 2103.pdf
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2103.pdf

Bob Clark