I think lightsabers should be left unexplained in Star Wars (they have a nice, mystical fantasy quality about them, like the Force, that would be ruined if you tried to explain it by saying, oh, something like it's actually a blade made of mitichlorian waste products)...
Having said that, I can't resist playing the sci-fi theoretical science guessing game... so here's my theory:
A saber is some kind of super-hot plasma that's kept in blade shape by a powerful electromagnetic field. That way you can lengthen and shorten the blade and is passes through anything without a magnetic field... which, of course, excludes other lightsabers... so perhaps the magnetic fields on lightsabers are naturally repellant (like the negative sides of 2 magnets) and that's why lightsabers bounce off each other... after all, magnetic repulsion is what makes everything else solid (otherwise, whenever you touched anything, you'd chemically bond with it)...
Michael Chang Gummelt