It seems as if Google dropped the Wikipedia layer from Google Maps a few weeks ago.

A Google Maps Help thread has dozens of complaints from Google Maps users that the Wikipedia layer is gone.

The layer provided additional content from Wikipedia about location specific data on the map. Now, it is no longer available.

In fact, if you look at the help document about the available layers, you will not see Wikipedia listed there. The current options include:

Traffic
Photos
Labels
Webcams
Bicycling
45º
Terrain
Transit

Google has not yet responded to the complaints in the thread. But it does seem the Wikipedia layer is officially gone from Google Maps.

Forum discussion at Google Maps Help.