There is a recently launched ETF, the Powershares Currency Harvest Fund (DBV), that look interesting. Details here: http://www.dbfunds.db.com/dbv/index.aspx
They claim that the strategy would have produced attractive returns with relatively little volatility over the past 10 years. It supposedly works by going long futures of the three highest yielding currencies and short the three lowest yielding currencies within a universe of 10 liquid, developed country currencies. Mechanically, it is done via currency futures collateralized with T bills, rebalanced quarterly.
I haven't had a chance to to all the reading yet, but it looks interesting. I have a number of concerns that will need to be satisfied before I even consider investing in this thing, but I would be interested in other opinions.