If A hits B and B then intending to hit A but hits C instead, then if B would have been justified in hitting A, B will not be criminally liable for hitting C even though C was innocent. However, if B was not justified in his response, then B will be both potentially liable criminally and civilly to C.
Thus, if Artest would have been justified in hitting the actual fan that threw the cut, then he will not be liable to the actual fan he hit regardless of the innocence of such fan (but his mistaken identifying of the fan must be a reasonable mistake).
In this case, however, Artest faces an uphill (but not impossible) climb of arguing self-defense and thus will likely be liable to the innocent defense.
Ahhh.... the days of law school!
Only Johnny Crackring would argue that Artest acted in self defense. "If the beer cup flies, the thrower dies!"
Seriously though, look at the guy he attacked. What, maybe 5' 10, 150# tops? Where is the additional "danger"? And even if there is additional "danger" does its threat rise to the level that justifies the response by Artest?
He clearly crossed the line here and he'll pay.