I've read this post an got very excited. I'm new to this trading business, so it was all interesting new stuff for me.
So I researched a matter of EMA a little, and then began testing.
I've got 2 month / 30 min period stats from bitcoincharts for bitcoin/usd price. Then I've wrote a soft that calculates ema based on this data, essentially emulating trading bot. Using your strategy with 10/21 this what I've got:

Starting price for btc was $925. I've started with 1 btc or 925 usd for simplicity.
There were 128 virtual trades made, when ema lines where crossing. After that I've ended up with 1.2085 btc == $731, when last price for btc was $605.

Kind of disappointing result. One could just take $925, buy btc for $605 and get 1.53 btc. So, what's the point?