I saw some sites on google being index without even submitting it to googlewebmaster tool. Although i am not sure about the ping you mentioned if it could be factor.
Google indexes almost every site with some content, but if your site has links from twitter, facebook, g+ or other websites the result will be faster.
Pinging your site alone is not sufficient for getting Google indexes. I have got links from twitter, facebook, G+ and other web sites to enrich my site's google indexes.
I saw too that sometimes Google index sites without adding their url to Google. For example when you create an own blog on wordpress on their hosting, your site will appear in search results without add url to google.
It would be really odd for Google to index websites just on pinging. The best way – is to leave a back link on any website that is already indexed.