How many languages?

Today's Boston Globe asks (and answers) the question "How many different languages are there?" The answer it gives [it estimates the number at about 6,900] is possibly a tad simplistic, as it doesn't define what is a language, and so it leaves itself open to interpretation.

However, the article does offer some interesting titbits, such as "About half of the world's languages are expected to die out within the next century. It is estimated that the last fluent speaker of a language dies every 10 days or so. Almost 550 languages have fewer than 100 fluent speakers still alive." It also delves briefly into Noam Chomsky territory, looking at basic linguistic structures, but obviously has limited space with which to do the subject any justice.