Unfortunately there's not an easy answer to this one.
SirCaustic's opinion is no more or less valid than flipflops because neither sat each other's paper.
There is something called the Flynn effect which shows that in general people are getting cleverer
http://en.wikipedia.org/wiki/Flynn_effect
However it seems unlikely that that on its own is enough to explain the trend.
Whilst Waldo's point about the Bell curve this has gone when flip-flop sat his in 89.
I'm unconvinced by the private schools argument because I don't think the rise in children in private schools has gone up that highly (but I could be wrong)
It would be very interesting to compare a maths paper from 20 years ago with one from today
Anybody know where to find one?