An official announcement from Napoli is still in the wings, but it looks like Ivan Strinic has signed:
That's the club's official twitter account showing the Croatian fullback off in training, which he would only be cleared to participate in if everything was done and dusted. The report linked to the club's official site indicated that Strinic partook in the club's full training session today, and said that he would be presented to the media "in the coming days." Given that the club has an open-to-the-public training session tomorrow, that seems like the perfect time to do it.
Strinic took part in the six-a-side mini-tournament that Napoli sometimes close their training sessions with, but his team didn't emerge victorious. Today it was a squad of Marek Hamsik, David Lopez, Josip Radosevic, Christian Maggio, Raul Albiol, and Antonio Rosati (who apparently still exists!) that took the win.
Barring anything unforseen, Strinic should be part of Napoli squad that will hit the road to play against Genoa on January 6th. With Faouzi Ghoulam leaving for Algeria's training camp ahead of AFCON soon, getting Strinic up to speed as soon as possible is vital, so seeing him in training already can only be a good thing.
Update: The Croatian Football Federation just posted a news item saying that Strinic has moved to Napoli, but even they acknowledge the lack of formal announcement from the club. So... official-ish?