From the New York Times' Economix Blog, this nice picture which shows that the US is not exceptional in the relative decline of manufacturing in employment. Why? A big part of it is that more and more often, manufacturing is happening in developing countries instead of developed. Another reason is that manufacturing itself is becoming relatively less important as a part of GDP as technology has made services more important.
This trend will continue in my opinion as what will become particularly valuable in the 21st century economy are technologies that make us more efficient.