Why have my funds in Bitcoin Block Explorer's online wallet decreased in value?
Bitcoins are traded on many different exchanges.  The prices are varying  constantly just like any currency.  As an example:  at this time three rates are Mt.Gox - $143.0983; CampBX - $125.8000; and BTC-E - $124.0000 (all USD).  As I learn more, and become familiar with the exchanges, I may try some arbitrage.  I have a feeling there are problems, or people would be doing it now, and all the exchanges would be more in line with each other.