I hate steel as much as anyone else, and I doubt very much that lead pellets are a serious problem in upland hunting. However the effects of ingested pellets on waterfowl have been known since the 1920's; large numbers of ducks died from ingesting lead from sandy bottoms before the lead ban. There is no question that condors and a lot of other scavenging birds are dying of ingested lead (to say nothing of the rodenticides that are very widely used in agriculture). Apparently the biggest problem for condors are ground squirrels shot with .22's. Currently, every condor in California is trapped annually and undergoes a process like dialysis to remove lead from the bloodstream.

For anyone interested in the science: http://www.peregrinefund.org/subsites/conference-lead/2008PbConf_Proceedings.htm