Actually Lucas in one interview after Ep3 came out, said that the mention of Plagueis served two roles. One, it was to make the audience decide on their own if Plagueis was some kind of super Sith Lord more powerful than Palpatine who had possibly created Anakin. Two, it was maybe a story Palpatine said about himself, and that he was in fact Plagueis. My take on it is that Plagueis is in fact Palpatine's master and that Plagueis must have had this knowledge of using the midichlorians to create life and didnt tell Palpatine about it. As a result, when Palpatine decided to become the master he killed his teacher and more than likely when going over his teacher's notes and so on discovered that his master had in fact found a way to achieve immortality. Palpatine himself said to Anakin after Mace got thrown out the window that 'to cheat death only one has attained that power but if we work together..' so in other words, he more or less confirms that Plagueis was in fact his master.