[pygorex]

For a laser intensity greater than some critical value, pair production is generated via a 'break-up' of the vacuum polarization. While the electric field strength necessary to accomplish this is immense, to say the least, recent experimental advances have raised hope that lasers may soon achieve field intensities on the order of this very critical field intensity.

As a layman this seems to me to be the crux of the matter. First, there's the the energy costs required to generate a laser that will in turn generate anti-matter from the "void". Will the energy released by the anti-matter fuel cover the costs of generating the laser?

If so, this technique would also provide a way to generate a perpetual source of energy. The laser costs X energy but produces an explosion of >X energy. Who says this energy can only be used for propulsion? You could feed some of this energy back into the laser, covering the continuous operating cost of the laser, then dissipate or distribute the remaining energies.

Of course, I'm just a layman, so I might be completely off base on proposing this perpetual motion machine :)

There's also the problem of sequestration and storage. How do you keep the proton-antiproton pairs from immediately colliding with each other? Isn't this the basis of the fuel? Colliding matter/anti-matter pairs? For long-term storage how do you keep the anti-matter suspension intact until you're ready to ignite it?