Failing to act and follow up on cease and desist letters against potential infringers of a patent may preclude enforcement of the patent. This is particularly true when the patent owner does not begin litigation or execute a license for the patent and, during the intervening time, the potential infringer takes action to build its business in reliance on the patent owners failure to act. These issues were addressed by the Federal Circuit in a recent decision, Aspex Eyewear Inc v. Clariti Eyewear, Inc., 605 F.3d 1305 (Fed. Cir. 2010).
For further details, see a discussion of Aspex.