Diplotaxis sp..
Size: approx. 10-15 mm. Photo: 04/09/12, 10 pm.

This beetle belongs to the genus Diplotaxis and is possibly D. liberta. There are over 200 other known species in North America and Mexico with the largest number found in the southwestern US and Mexico. Adults are nocturnal and feed on the leaves of various plants.  
bug guide (this photo):