Hate to cast doubt on the fount of all knowledge - but are you sure JP?

On this one, yes.

Anti-particles can be seen as particles with opposite time line (ie in feynman diagrams)- but surely not as particles with negative mass / negative energy. Dirac's initial conception of the positron was as a gap in the negative energy (and charge) sea - a gap in the negative energy sea is positive energy

It would seem that way, but I'm quite positive on the fact that the antiparticles are solutions to the Dirac equation with E<0. This comes from the particles essentially satisfying E

^{2}=m

^{2}c

^{4} in their rest frame. If you take the square root on the left-hand side, you get both positive and negative-energy solutions. The negative energy solutions are anti-electrons.

The other reason I'm sure is that this is often the reason that quantum mechanics describes the time evolution of particles by a term that contains Et, the product of the energy times time. If you reverse the energy, this term "runs backwards" in time, meaning that E=1, and t=1 looks the same as E=-1, t=-1. This is the basic idea behind negative energy particles running backwards in time.

I think the term "hole" comes from the idea that if you think traditionally about energy and negative energies are allowed, an electron should keep emitting photons and dropping to the lowest possible value, which is unbounded in this case. So Dirac had to propose that all the negative energies in the universe were usually filled up, so the electron couldn't drop below zero. However, if there is a vacant spot in the negative energy sea, this acts as a positron with negative energy, but positive charge. I never really learned much of the details of the Dirac sea because it has fallen out of favor (clearly thinking of space as filled with infinitely positrons has issues--namely with infinite energy at any point in space). More modern interpretations still deal with negative energy, though, in the senses I mentioned above, and in particular with it inducing a "backwardsness" in time. Again, I think most physicists don't consider negative energy to be physical, just a useful mathematical tool. I believe that generally positrons are thought of as positively charged antiparticles rather than negative-energy electrons, but the mathematical trick is still interesting.

Here's a link I found to one of my graduate textbooks on the subject, which has parts available by Google books. Check out pages 576-577, which describes it a bit (thankfully without relying on equations!):

http://books.google.com/books?id=2zypV5EbKuIC&pg=PA581&dq=shankar+principles+of+quantum+mechanics&source=gbs_toc_r&cad=3#v=onepage&q&f=false