Some of my recent Google searches, undertaken after having read the astonishing book, The Da Vinci Code, by Dan Brown (which I will review later):
The Last Supper dagger 
Council of Nicaea
Gospel of Philip Thomas Mary
Another wierd coincidence. While writing this post, this article in the WaPo just came online, summarizing the DaVinci Code, as well as directly addressing some of the threads above. This is a fantastic article!
I think what really intrigues me is the Leonardo connection. His paintings are indeed full of code. Look for recurring images of the "pointing finger", which appears in Adoration of the Magi, The Last Supper, and Madonna on the Rocks. As well other symbols like a claw-like hand, a slashing hand, and deliberate obfuscations of gender (the most prominent example of which, is his most famous painting of all time).
 Yes, there is a dagger in the Last Supper. Varying interpretations of the painting claim it is held by "Simon the Zealot" or by Peter. But I've gone over high-resolution photos of the painting, and the hand holding the dagger is disembodied. It does not correspond to any human at the table. Though, it would certainly make sense if it was held by Peter... see the Gospels of Philip Thomas and Mary search above.
And is the figure at the right of Jesus really John? John seems to have a very ... fair demeanour. And long red hair. Remember Leonardo daVinci was a master, and knew how to portray the difference between the sexes. I can't look at that figure and see it any other way, but then again I am free of the pre-concieved notions that may dominate some orthodox Christian viewers of the painting.