For netbooks, AMD's configuration combines a SB750 southbridge with a Ontario. Since the wireless device is directly hooked up to the APU's single PCIe link, there are improvements in power management, as the southbridge can go into an idle state without sacrificing connectivity.
For notebooks, the plan is to pair a SB750 southbridge with an Ontario or Zacate, with the additional use of a four-lane PCIe connection to a discrete GPU. However, this sacrifices some power management savings by hooking the wireless device to the southbridge. AMD's logic is that in a netbook, users would be less likely to need a discrete graphic solution. By moving wireless connectivity up to the APU, the southbridge only needs to deal with I/O devices like the keyboard, touchpad, USB devices, and flash media. Given that mobile users are less likely to use USB devices and flash media while on the road, the SB750 only has to transfer small bits of data from the keyboard and touchpad, which translates into higher power savings.
Everyone can benefit from power savings, and in a world where we leave our wireless connection active, the ideal situation would be to always have the wireless device hooked directly into the APU. However, this is not possible if you are using a discrete graphic solution in a x4 configuration. Remember, there are five PCIe controllers off the APU, and one is reserved for the UMI link. The other four are intended for peripherals. For discrete graphics, you can either use a single x1, x2, or x4. So in practice, it is possible to connect a discrete graphics chip with x2 and still simultaneously have two x1 connections available. Meanwhile, all "Hudson-M1" southbridges come with a UMI connection to the APU (Ontario/Zacate) that is based on a single x4 PCIe connection; probably with some aspect of proprietary signal handling.