A recent study by Hauk et al. (2008) attempted to address this "imagery" vs. "meaning" ambiguity. They designed a decent study. There was a set of action words (e.g. ‘grasp’, ‘limp’, ‘bite’) and a set of non-action words that had imageable attributes (e.g. ‘snow’, ‘blond’, ‘cube’) and they looked for regions that showed word frequency effects that were specific to one category versus the other. The logic was that imagery processes should not so sensitivity to word frequency effects, as word frequency effects are associated with lexical-level processes. We could quibble here -- e.g., they didn't control concept familiarity -- but let's not.
So what did they find? Did they find category-specific frequency effects? Yes! They were able to identify one region that only showed frequency effects for action words and a different region that only showed frequency effects for non-action words. Does this mean that motor cortex, which presumably is coding action semantics, really is coding action semantics? Well, no. It turns out that the frequency effects were in the temporal lobe, not the frontal lobe:
If you don't read this paper carefully, it is easy to miss this little fact. For example, the abstract states:
we corroborated previous results showing that action-relatedness modulates neural responses in action-related areas, while word imageability modulates activation in object processing areas.
This is not a false statement, it's just that the "action-related areas" they are referring to are in the middle temporal gyrus. They go on to write,
we suggest that category-specific brain activation reflects distributed neuronal ensembles, which ground language and concepts in perception-action systems of the human brain
Again, based on their findings, these "perception-action systems" don't seem to involve motor cortex.
The findings of this study are interesting, but they seem to support the view that even action concepts rely more on posterior brain networks than frontal, motor-related brain networks.
Olaf Hauk, Matthew H. Davis, Ferath Kherif, Friedemann Pulvermüller (2008). Imagery or meaning? Evidence for a semantic origin of category-specific brain activity in metabolic imaging European Journal of Neuroscience, 27 (7), 1856-1866 DOI: 10.1111/j.1460-9568.2008.06143.x