I certainly think it's suggested, especially since Palpatine gives Anakin that meaningful look after saying it.
Whether "suggested" means suggested by Lucas as the truth or suggested by Palpatine in order to further bait Anakin will probably never be resolved - I' m looking forward to anything Lucas will say about this in future interviews.
It comes from the novelezation, after Sidious tells Anakin he's a Sith, Anakin asks if Plagueis was his master and he says yes, but you don't even need that. It comes from the theatre scene, when Palpatine is telling the story of how his apprentice and he gives a wickedly evil, reminiscent look when he says he was killed in his sleep.
Originally posted by Jedi Merc
the force is anakins father, there are other threads related to this
I wondered about this but then I thought... Nah, one dollop of cheese to many. I don't actually read EU so maybe some one can fill me in on this. I don't like the idea though. As for Sids as Ani's Daddy, well maybe GL toyed with the idea and then quit. For me that would be one parallel with the OT to many. To lazy and predictable.
Originally posted by Delta 62
Sidious couldn't have created Anakin through the Force. He told Anakin that only Plagueis had mastered the technique for creating/saving life..but that together they might be able to.
Well he also said Plagiues taught him everything he knows.... but I do NOT believe either of them created Anakin