Huge influx of forex in the 1st quarter of ‘07 - $135.7 billion as reported by PBOC.

More from the FT:

Just under half of the increase in reserves can be explained by two
factors: the trade surplus for the first quarter, which was $46.4bn,
and foreign direct investment (FDI) of about $15bn.

The trade surplus, of course, has been the major culprit for many years, and all the analysts out there who were commenting over the past few days on the aberrant March numbers have uniformly predicted that the long-term trend will continue.

[O]n top of the trade surplus and investment, another $25bn of funds have flowed into China every month so far this year.

The
most common explanation offered for the surge in funds is the unwinding
of currency swaps that the PBoC is said to have arranged last year with
Chinese commercial banks to keep funds offshore.

Some gigantor IPOs and investment deals made over the past couple years have meant huge piles of cash sitting offshore, with domestic banks just waiting for the green light from Beijing to bring the money in. Looks like the light turned green, at least partially.

Another possible factor is the slow but gradual strengthening of
China’s currency, the renminbi, which may be attracting “hot money” to
China.

This one is kind of hard for me to take too seriously as an explanation for a dramatic increase, although the article does not explicitly imply this. Everyone has known about the need for the RMB to be revalued for a number of years now, and the hot money has been coming in as a result over quite a long period of time.