I just ran across the following old analysis of SPS and
alternatives by the US Office of Technology Assessment,
in 1981. Does anybody on the list remember this?
A lot has changed since 1981; even then they projected
a timeline of 2005-2015 for SPS to become practical,
but they were expecting the US to be independent of
foreign oil and busy building breeder reactors (and fusion
plants) by now too... And while there were worries about
sulfur dioxide and acid rain from coal, I didn't notice
any mention of CO2!

See:

http://www.wws.princeton.edu/cgi-bin/byteserv.prl/~ota/disk3/1981/8124/8124.PDF

Arthur