Q:  Do I need to reinforce the supporting cabinetry in order to accommodate the weight of a concrete countertop?

Generally speaking, no, when using a quality cabinet.  If the supporting cabinets are particleboard, vertical plywood reinforcements may be necessary.  A 1.5" thick standard concrete countertop has an approximate weight of 20 pounds per square foot and will be dispersed over the entire cabinet structure.  LinkClick.aspx