Russet spotting is strictly a visual defect. It is due to formation of colored compounds from simple phenolics and is generally induced by ethylene exposure in the postharvest environment. It is in no way harmful to eat lettuce that has those discolored spots.  That being said, you want to clearly identify it as russet spotting and not some other problem (like bacterial).  For photos of this disorder, click on the photo album associated with the lettuce produce facts on our website: http://postharvest.ucdavis.edu/pfvegetable/LettuceCrisphead/.

-Marita Cantwell