At 3% equity, he would need either a really, really big hit or a lot of big successes to become a billionaire. From YC's modus operandum, I think pg cares more about shaking up Silicon Valley than he does about maximising return on investment.
either way, that would be cool to see pg become a billionaire. I wonder how he would shake up philanthropy? He might be able to find a better charity model than what currently exists today..
And, with the standard Malcolm Gladwell disclaimer, http://www.gladwell.com/2006/2006_02_13_a_murray.html seems to indicate than most homeless people aren't homeless for very long.