It seems kind of harsh to me. The way I see it, if a fan gets on the court, they are fair game. In the case of Jermaine O'neal, the fan was on the court, and he punche dhim. Artest went into the stands as did Jackson. Jackson's punched actually looked worse to me than Artest's while in the stands. The guy Ron Artest punched who was on the court is fine by me. Everyone wants to villify the players, but the fans need to know how to react all the same. Those punks who got on the court deserved to be pummeled.
In baseball, if a fan gets on the field, the players have tackled and punched them without a problem, the same should happen in NBA games.
I used to be a Pro Wrestler, and we had, what I think, was good security surrounding the ringside area. However, on occasion, some moron would hop the railing slip past security and get in the ring. At that point, as a performer/athlete, you have no idea what these maniacs will do, so you defend yourself and subdue them anyway you can. Monica Seles was STABBED by a supposed fan. She had no time to call for security. Any fan in any sport that wants to step foot on the playing surface should be well aware that their face is most likely going to be caved in, and rightfully so.
Having said that, I think Artest should have been suspended, but not for an entire season. A precedent had been set before with Rodman, Maxwell and others. Given the fact that there were no major injuries and the most likely event of lawsuits and possible criminal charges, I think this punishment is too harsh. I would be shocked if the Player's Union doesn't fight this.