I like this one a lot.  Simple but bold. 

First I thought this flag is too political, but then again most people don't know what it means.  Then I thought it is fitting gold and black.  Bitcoin is stateless after all, and used for trade and exchange.  I do like it.  If you wanted to be super subtle you could take the BTC off and just change the yellow to the yellow color that is often used with bitcoin, it's an almost orange.  Here is the color