I just bought a vacation condo in Playa recently and purchased myself a cell phone. I went with the "Amiga" plan which is pretty popular there -- no monthly fee, can receive local calls for free, add minutes for outbound calls by purchasing phone cards. I've found the little phone store at the front entrance of Chedraui to be convenient and helpful. The phone cost something like 650 pesos I think and that included 300 pesos of time if I recall correctly -- a good deal in my book.
Chedraui is the grocery store a block south of Juarez and fronting the highway on the east side.