This compares reasonably well with the results of my survey (http://en.wikipedia.org/wiki/User:Carnildo/The_100): about 15% of all articles are sourced. My methodology was a little different, as I counted anything in a "references" section as a reference, while inline links were collectively counted as one reference.
I'm still working on a survey of biography articles, but preliminary results are that slightly fewer biographies are sourced, but ones that are sourced tend to have more sources.
Nice! Are you interested in expanding it a bit, perhaps to include grades in various categories like adherence to MoS, PoV etc? It would be very interesting to get a few people together and regularly take samples of 1000 or so articles, to see whether things are getting better, worse etc.
Steve