Again, the problem is generating enough antiprotons. As of now, the energy (and money) required to create them is far greater than the energy generated when they are annihalated via regular matter.
Just to give you an idea of today's technology, here is the current status of the Tevatron, the highest-energy particle accelerator operational today (although it will hopefully be superseded in a few years by the Large Hadron Collider):
http://www-bd.fnal.gov/notifyservlet/www
At the moment I'm typing this, there are about 821 billion antiprotons, and each one carries about 979 GeV of kinetic energy.