We can't, as I understand it, litigate for any infringement occurring after the patent expires, but litigation is still an option for infringment during the "look back" period. 

Remember also that there are allegedly many infringment notificatiins already in effect. Recall the "list" disputed between PTSC and TPL/Alliacense.

"35 U.S.C. § 286 Time limitation on damages

Except as otherwise provided by law, no recovery shall be had for any infringement committed more than six years prior to the filing of the complaint or counterclaim for infringement in the action."

"If a patent is infringed while it is enforceable i.e. before it expires, the patent owner can sue the infringer up to 6 years after the patent expires. This is important in a situation where the patent owner may not become aware of the infringement until after the patent expires."