I thought it was not a "defense" but grounds for a motion to dismiss...
here's a link to a case opinion in which standing was attacked at the appellate level:
http://4closurefraud.org/2011/01/30/...k-of-standing/
If the plaintiff lacks standing then the court never had jurisdiction over the defendant -- hence a motion to dismiss would be in order
I don't know if the plaintiff detailed their standing in their complaint ~ if they did not, then the court may find in the defendant's favor -- a big if of course