Indeed. Looking at this:
http://www.floatingsheep.org/2009/11/mapping-wikipedia.html
This is a similar mapping:
http://commons.wikimedia.org/wiki/File:Imageworld-artphp3.png
I think there is a huge number of notable topics that we have not yet covered. Sure, there may be fewer sources about central Africa, but what about China and South America? The areas most Wikipedians care about are well covered, so we don't notice the gaps. The meme that Wikipedia is finished and we only need to add new things that become notable is very dangerous.