DeLong (h/t Jazz):
In 1950, finance and insurance in the United States accounted for 2.8% of GDP, according to US Department of Commerce estimates. By 1960, that share had grown to 3.8% of GDP, and reached 6% of GDP in 1990. Today, it is 8.4% of GDP, and it is not shrinking.
The yearly values for GDP growth since the 50's are contained in a collapsing envelope, indicated by the green trend lines. These lines converge at about 1.8% in 2028. Make of it what you will.
Graph updated 2 Jan 2012.