Apparently in UK's Metal Hammer magazine Kerry King mentioned that Lars Ulrich had been contacting people from the above mentioned bands about getting a tour together.
Now the odds of this happening are slim-to-none, but just imagine how amazing that tour would be... and probably expensive. Plus who would headline? I would imagine they would all have to work it out so they played about evenly or there would be some infighting for sure from the get-go.
Also King already dealt a blow to Anthrax by saying:
"I said in this day and age, I know we had that time together, but how do you leave out MACHINE HEAD? There's better options than ANTHRAX and that's nothing against ANTHRAX, but they've just been in pieces for quite some time and that doesn't make as much sense to me."
Look for updates as they become available.
UPDATE #1: Scott Ian is game, in a recent interview he was quoted as saying the statement below.
"There's this big rumor that we're gonna do METALLICA, ANTHRAX, SLAYER and MEGADETH next summer in Europe, and this is supposedly coming from the METALLICA camp. So whether it's true or not, I'm really excited about it, and I'll say right now that we will absolutely do it. So we're in. [laughs]"