In researching the environmental footprint of my cellphone (Samsung Omnia i910), I came across this report by the EPA on the environmental impact of solders, comparing lead-tin (PbSn) to lead free formulations: http://www.epa.gov/opptintr/dfe/pubs/solder/lca/index.htm
There’s a lot to solder in electronics, and a lot to the solder itself. Different alloys melt at different temperatures, and flow in different ways, complicating the assembly process if engineers have developed components and boards around PbSn. Another complication is that lead has the fortunate ability to inhibit the growth of metal whiskers, which have killed components in a nuclear power plant,in the Galaxy IV communications satellite, and in cardiac pacemakers. http://en.wikipedia.org/wiki/Whisker_(metallurgy). the RoHS (Restriction of Hazardous Substances) directive allows for exceptions for critical components, but in a fortunate turn, major manufacturers are making their equipment toxic metal free even though excepted from restrictions.