It's called the Flynn Effect.
I think that class and rising standards of living + education might be part of it; but I don't really know.
Wikipedia:
http://en.wikipedia.org/wiki/Flynn_effect
Linked to in Wikipedia:
http://pespmc1.vub.ac.be/FLYNNEFF.html
SciAm article:
http://www.sciam.com/article.cfm?art...mber=2&catID=2
An explanation attempt:
http://www.fairtest.org/examarts/Winter%2001-02/IQ.html
A humorous religious correlation:
http://www.gnxp.com/MT2/archives/001527.html?entry=1527