Mechanism of Grignard addition to carbonyls
This reaction can be viewed as an example of a pericyclic (concerted) Ene reaction.  The ene reaction is actually fairly common in organic chemistry.   The following diagram details the mechanism as a concerted reaction with a cyclic 6 membered transition state involving 6 electrons (an "aromatic" transition state).



A note on the Ene reaction...

The Ene reaction is really just a variation on the Diels-Alder theme.  The Diels-Alder reaction is a concerted [4 pi + 2 pi] reaction, whereas the Ene reaction is a [(2 pi + 2 sigma) + 2 pi] concerted reaction.