Ripple XRP in circulation From the 100 billion of total XRP, 20 billion went to the founders and 30 billion to OpenCoin to fund development. The 50 billion remaining are supposed to be given away to promote Ripple. 

Is there any source of information tracking how many XRP have been distributed to the public?

Have any of the funds for development been sold?
this is pretty good article calculating how many XRPs are floating on the market

http://www.thebitcointrader.com/2013/05/ripple-is-now-tsunami.html