I have been the most stoic resistor to the notion of massive OS reservations. However, if all the rumours and noises are saying that, it's time to reconsider rather than indulge in denial. What exactly is a console OS? I mentioned Orbis's significant 512 MB OS footprint, and if that includes all the features of the PS3 optional functions that consumed significant MBs (things like Friend List taking 20 MBs, Voice Chat taking 20 MBs, all on top of the OS reservation), it's not such a huge chunk relative to PS3.
So what could 3 GBs be? Maybe there's 1 GB HDD cache, handled by the OS? Maybe enough for a webpage for in-game guides? There's going to be something taken up by Kinect and voice recognition. IIRC they are more data driven than processing driven.
In short though, I think it's time to remind ourselves these are consoles are the OS will be console-centric, serving needs that we don't associate with PCs and smart-devices.
Assuming the AMD APU has an ARM core because of TrustZone. There will probably be some overhead managing and running secure OS services. This may also include disk cache, security services for PSN/XBL, HDD and BR.
Then, the device drivers for sensors, assorted controllers (natural interfaces or otherwise), and wireless/second displays.
Plus user-mode layers and apps such as the web browser, in-game XMB.
iOS fit most of these functionality + apps under 1GB. Android did so under 2GB. So most of the extra memory should be for data. Perhaps LiveWall "panoramic view" buffers. For Sony's case, whatever their OmniViewer takes.