Am I getting completely nuts ?
It seems there are shells in this pics released on sol 105 by the microscopic imager of Opportunity !

Sorry, I made a mistake with the 'preview post' button...

Here is the picture

Those are some very shell-like structures, reminicent of micro-crinoid ossicles in some terrestrial rocks. However I would want to explore all abiotic possibilities before I even began to consider a fossil origin.
Somtimes minerals which crystallize in a cavity, even a very small cavity, will do so in a radial pattern. It's possible that is what is pictureed here and in the small fracture in the lower left of the image.

Given that the object you pointed out is almost perfectly round, wouldn't it be more likely we're looking at a half-berry (with wind blown dusts embedded in it?