I would never see it as an m43 replacement, although depending on how limited a set of lenses you use with m43, maybe it'd be something to look into. But I seriously doubt the sensor will be competitive with those in the new Olympus bodies. As an RX100 replacement, I'd be surprised if the X20 sensor will beat the RX100, but if it comes close to being as good, its a much different camera and might be worth consideration depending on your preferences. I personally didn't like the RX100 and loved the X10 and would love the X20 (although probably not as much as the LX7). It'll have a much faster lens than the RX100 except at the widest settings, and its a much much MUCH more tactilely pleasing camera to shoot with if you like making adjustments on the fly. This is purely my opinion of course, but its a pretty strong one...
-Ray