Some more info about AMD's Fusion has leaked on the Internet.
The cores, memory controller and the Hypertransport are located on one die, while the GPU is located on another, connected to the main package with AMD's own tech. This is not the only MCM that will come from AMD. Montreal is two Shanghai dies slapped together onto a Socket F+/AM3, so eight-core Opterons/Phenoms will be something that Intel already sells today - two pieces of silicon using a single socket.
AMD, Chartered and TSMC are all involved in manufacturing Fusion processors in one way or another, not just one.
AMD's Fab36 and Chartered will be in charge of manufacturing the CPU part of the package, while TSMC will manufacture the graphics part. This will significantly reduce the cost of manufacturing, since it will lower the risk of yields going awry. Neither of the companies in question has experience to produce a complete CPU/GPU being glued onto the same chip module.