You can now buy tickets for the archaeological sites of Pompeii (11 EURO), Herculaneum (11 EURO) and Oplontis (5.50 EURO) on-line. The tickets are valid for one day. There is also an integrated ticket for those three sites plus the villas of Stabia and the Antiquarium at Boscoreale (20 EURO) which is valid for 3 days. See http://www.pompei.napoli.com/ to buy the tickets, which you must then print out and present to the site ticket collectors.
This is the SAP press release.