Today a source familiar with AMD product planning has come forward to deny such rumors by calling the 18W and 25W TDPs for Ontario "not reasonable and well outside the margin of error". Although the source did not disclose any actual power consumption details regarding the first Fusion chips, X-bit labs claims to have seen documents describing their higher-end siblings (Llano) as Phenom II-class x86 chips with high-end graphics cores that are meant to fit into a 20W thermal envelope. Thus, having them overlap in terms of performance or power consumption wouldn't make sense.
Ontario-based products are expected to feature one or two Bobcat x86 cores merged with a graphics core capable of DirectX 11 rendering. Although AMD hasn't shared much beyond that, it is has previously been implied that Bobcat itself is sub-1W-capable and will be a direct competitor for Intel's Atom.