Yoda will be defeated by Sidious in Ep3
OK. More specific. Yoda faces off in a *physical confrontation* with Darth Sidious and is soundly, roundly beaten. It's not metaphorical any more than 'you will see why Yoda is the MASTA' was (I don't honestly know if lightsabres are involved; what I gather is that Sidious *doesn't have* a lightsabre, although that is older information.)
In fact, Yoda is very nearly DESTROYED but for the sudden intervention of one of our heroes, and no I'm not going to tell you who; anyway that is the sort of detail Lucas could change, since it is unimportant (the pacing of action at this point will determine these details, much as it determined who hypnotised the Reek). Also, at this point as everyone rightly points out, it is loose information and nothing is certain because it is too EARLY.. except for the fact that this scene has been written already!
Yoda escapes by the skin of his teeth. There is a lot of stuff happening at the same time at this point; many of the plot events people expect to occur at the end of Ep3 in different locales or times actually occur in the same place, more or less all at once.