Update: Sputnik sez, "Where can I see this without getting my fool self electrocuted? As always, YouTube to the rescue!"
Update 2: Mike sez, "Years ago when I worked at Digital Equipment Corportation, this hilarious 'research paper' from DEC's Western Reseach Lab was widely circulated. Entitled Characterization of Organic Illumination Systems, it details arcing pickles and other assorted vegetables."
Update 3: Wayne sez, "I conducted this experiment years ago as my final high school chemistry project, trying to figure out why only one end of the pickle glows. I came to the same conclusions mentioned about the sodium, but was unable to figure out the polarization. Useless-knowledge.com makes reference to this scientific mystery of pickle polarization:"
Why does only one end of the pickle light up and glow? Look at the amazing electrical storm jumping through the pickle. (Results are best viewed in a dark room. This is better than Star Wars! Don’t worry the pickle will make all the light you need.) Unplug the pickle; reconnect the wires on the opposite side and it still only glows on one end. There is yet no definitive scientific answer to explain the polarization of a pickle connected to AC current.
Update 4: Dan sez, "Years ago, I figured out that if you buy one of those hot dog cookers that runs current through the meat to cook it, you could use it as a (somewhat) safe version of the glowing pickle lamp."