Note that, as several people have pointed out but seems to have been lost in the noise: this only affects the main page.
Accesses to www.wikipedia.org/wiki/Foo are redirected to en.wikipedia.org/wiki/Foo so that existing article links still work. (I just tested this: it does work).
I believe that this is something long past due. While the English wikipedia is still the largest, a number of other languages have reached the point where they are substantial, useable encyclopedias. It is only right that the project's main page be an index of the available Wikipedias.
-Matt