The US Supreme Court held in Limelight Networks, Inc. v. Akamai Technologies, Inc. that a defendant is not liable for inducing patent infringement if no one has directly infringed the patent.  Thus, when claims are being drafted, particular importance should be placed on assessing whether more than one entity be responsible for providing different steps or elements of the claim.  The US Supreme Court opinion can be found here Limelight Networks v Akamai Technologies