So what is light field photography, and why do you need a plenoptic camera?
Look carefully at the three pictures above. They are not three different pictures. They are the same image, refocussed after the picture had been taken.
Yes, after.
So that's why you need a plenoptic camera. Or at least why you want one - they are just at the research stage at the moment and a bit low-resolution. Put it on your Christmas list for 2010.
As to what light field photography is, I'll leave that to the experts at Standford University. They've got technical explanations, and more photos and movies taken with their prototype plenoptic camera - some are really astounding!