As far as I can tell, this is an unsubstantiated rumor.
I'm not too worried about it. If the pols go after 401k's directly, DC won't have enough rope, torches, or pitchforks to go around.
Now - what I really see happening....eventually....is DC doing away with 401k's. Or doing away with the tax breaks associated with them (virtually the same thing). Particularly if its done on a "voluntary" basis...I could see it happening, sooner or later.