To some extent, they do it at all of their hubs (SEA, SLC, LAX, MSP, DTW, JFK, ATL). However, they have differing numbers of cars and drivers (SLC only had one car last time I asked) and differing numbers of potential passengers/riders (ATL has more cars and drivers, but also more passengers vying for the opportunity.

SEA seems to have the best odds overall - at least based on my experiences. I have had 7 Porsche rides there now, and one at LAX.