I agree that these are long reads but really interesting:
http://optics.org/indepth/2/7/3
http://rohs.exemptions.oeko.info/fileadmin/user_upload/Stakeholder_comments/Exemption_13_several_industry-stakeholders_27_March_2008.pdf
Via: http://www.dpreview.com/forums/post/52159193
In short, lead-containing glass is still better optically, less temperature sensitive, more stable and actually cheaper to make. The use of lead in some optical appliances is actually exempted by RoHS directive but the adoption of lead-free glass in photographic lenses is yet the trend and the future of the industry.
Btw, I am glad that I still have some of those toxic Pentax glass that are lead-made, including some of the best F, F*, FA* and FA Limited lenses, which are now used on my Canon 5D3 FF, examples here and here! :-D