I discuss the current status of "trans-Planckian" effects in inflationary cosmology arising from the breakdown of classical spacetime below the fundamental length scale associated with string theory and/or quantum gravity. I focus on the primordial perturbations produced during generic "slow roll" inflation. I review open theoretical issues, before turning to a specific "case study" to show how a minimum length might modify the primordial spectrum. I then show how these modifications would be constrained by high precision Cosmic Microwave Background experiments, and assess the prospects for developing tests of string theory derived from observational cosmology.