I was a member of the UK Club (the oldest in the world by the way) when I lived in London and had some great events, both big and small. The International Event is generally very well organised though quite pricey.
I hear that the official celebrations may end up being split as follows (no official news yet): the Maserati International held in September by the Maserati Club Italia and a invitations only event organised directly by Maserati for a few owners.
As far as I know Club Tridente in Spain is no more.
The chap who is setting up the club in Spain is based in Barcelona. He doesn't speak English though.
Hey stradaman, a chap I know is setting up an official Maserati Club in Spain, endorsed by Maserati SpA. Let me know if you want to be put in touch with the guy.
Milan now has its own version of the Congestion Charge, plus lots of one way streets et al. Don't bother driving in, best to leave the car at a metro station and ride the metro in, eg in Sesto FS (the last, northernmost stop, on the M1 Red Line).