Ferrari reportedly used riblets on the undersurface of their front-wing at the Turkish Grand Prix last month. At the time, Autosport's Gary Anderson claimed that the intention was to "reduce the airflow-separation problems and make the wing work more consistently."
Coincidentally, in late 2009 a group of researchers from the University of Southampton published a paper in the Journal of Fluids Engineering, entitled Flow Separation Control on a Race Car Wing With Vortex Generators in Ground Effect. This paper contained the results of an empirical investigation into "flow separation control using vortex generators on an inverted wing in ground effect." In particular, the authors claim that:
"The counter-rotating sub-boundary layer vortex generators and counter-rotating large-scale vortex generators on the wing deliver 23% and 10% improvements in the maximum downforce, respectively, compared with the clean wing, at an incidence of one degree, and delay the onset of the downforce reduction phenomenon. The counter-rotating sub-boundary layer vortex generators exhibit up to 26% improvement in downforce and 10% improvement in aerodynamic efficiency at low ride heights. Chordwise pressure measurement confirms that both counter-rotating vortex generator configurations suppress flow separation...This work shows that a use of vortex generators, notably of the counter-rotating sub-boundary layer vortex generator type, can be effective at controlling flow separation, with a resultant improvement in downforce for relatively low drag penalty."
Note that Ferrari only appeared to employ their riblets on the undersurface of the front-wing, rather than the undersurface of the rear wing. This is consistent with the Southampton University study, which specifically involved the control of airflow separation under ground effect conditions.