So with the upcoming release of Assassin's Creed 2, I popped the original game into my 360 earlier in the week to refresh the story in my head.
I couldn't believe I didn't notice this before, but the symbol that the Hashsashuin use to identify themselves looks very similar to the Freemason's Square and Compasses.
Maybe I'm out on a limb, but I personally found the similarities striking. Especially given the secretive nature of the Masons themselves.