Bitcoin exchange Kraken has undergone an audit performed by Stefan Thomas and has proven that not only does it have 100% reserves but it actually has more than 100% reserves (between 100% and 100.5%)
Kraken's site: https://www.kraken.com/?setlang=en-us
and the full story: http://www.coindesk.com/krakens-audit-proves-holds-100-bitcoins-reserve/