Bitcoinica now publishes all the open interest data since January 20, 2012.
Open interest is often used in futures market as a technical indicator. Since Bitcoinica does not always have equal long positions and short positions, we only take the less one as the value of "open interest", so that the data will reflect exactly how many Bitcoins are being involved in both long and short positions.
For example, the open interest at this moment is 54600. This means that there are
at least 54600 BTCs of long positions and
at least 54600 BTCs of short positions on Bitcoinica.
Available at:
http://charts.bitcoinica.com/open_interestSample (2012-02-19 Open Interest):
