For all of those wanting to know about the shields on the Ferrari rear brakes, here is some information, straight from Formula1.com.
Ferrari 248 F1 Rim Shields
Ferrari first introduced these at Imola, to help extract the heat generated by the rear brakes. The lip of the shield separates the hot airflow from the rim walls, helping to stabilise tyre pressure by limiting the effect of the brake heat. In Turkey the concept is pushed further, with a wider lip section reducing the size of the central air vent. This improves air extraction still further by decreasing turbulence generated by airflow close to the wheel. It is also slightly imprves aerodynamic efficiency in this area.
Ferrari F2007 - Rear Wheelnut Extension
Last season, Ferrari adopted shields to cover the rear wheels rims. These additions meant the mechanics could only access the wheels with their wheel guns through a small central hole. To improve the timing and precision of this operation, the F2007 has been fitted with a wheel nut extension, which enables the mechanics to fit the wheels gun onto the nut more easily. This modification has already improved the timings of the team's pit stops.
Ferrari F2007 - Modified Rim Shields
As discussed previously, brake fade is a problem in Monaco. Less grip from the 2007 tyres forces brake bias rearwards, and cooling the rear brakes is difficult. To avoid problems in the 78 lap race, Ferrari modified their rear wheels' rim shields, giving them wide vents to allow the heat generated under braking to dissipate more efficiently. The vents lessen aerodynamic efficiency by adding drag, but this is of little consequence on the slow speed Monto Carlo circuit.
Hopefully, this will will clear up the questions about the rim shields on the rear wheels of the Ferrari.