Over at Lingua Franca, fellow Language Log author Geoffrey Pullum has an excellent article entitled "There Was No Committee".
Here's a key paragraph:
Some people talk as if Mandarin Chinese was gaining on English. It is not, and it never will. A Tamil-speaking computer scientist explaining an algorithm to a Hungarian scientist at a Japanese-organized scientific meeting in Thailand calls on English, not Chinese. Nowhere in the world do we find significant numbers of non-Chinese speakers choosing Mandarin as the medium for bridging language gaps. There are no signs of that changing.
As to why Mandarin is very unlikely ever to displace English as the world language, it's the writing system, my friends.
Now, if ever a true digraphia were to develop in China, Mandarin might have a fighting chance.
But I doubt that will ever happen. It seems that most Chinese people would sooner learn English than use Romanization to write Mandarin.
[Hat tip John Rohsenow]