According to the Directorate-General of Budget, Accounting and Statistics, Executive Yuan, over the past six years, the mean income of the 700,000 households with the lowest income just fell from NT$52,820 to NT$34,866 from year 2000 to year 2006. Thus, their mean current income is less than NT$3,000 per month.
Meanwhile the mean income of the 700,000 households with the highest income went from NT$1,621,747 to NT$1,741660 from year 2000 to year 2006. Thus, their mean current income is over NT$145,000 per month.
Meanwhile, the GDP is expected to grow by 4.14% per annum.
The M-shaped society is a reference to a society where the income curve has two peaks, one in the lower middle class, and one at the top, a trend noticeable in many industrial societies. Note that the DGBAS (stats page) data show that Taiwan's poorest households actually lost roughly 40% of their income since 2000.
A number of things contribute to this trend, including factories moving to China, but weak labor unions in Taiwan are a major problem for the working class, as Kerim at Keywords has pointed out. The implications for the island's politics of falling relative incomes in the middle and lower classes are obvious.