Winreview reports that Windows 8, which Microsoft made available on Microsoft Connect site to OEM's will have the option to run Desktop Window manager (DWM) using software acceleration, at least in pre-release builds -- meaning that "older and budget machines with little graphics acceleration will still be able to run Aero in a virtual environment."
Winreview had set up a machine in a virtual environment, which's able to run Aero using only software rendering and a few MB of video memory. He is quick to point out that we had a similar situation with early builds (CTP builds) of Vista: The version that ran on virtual PC was capable of software rendering, but the final version was not. Some effects, like the blur-behind effect, that Aero implements were considered too resource-intensive to leave it up to software processing.
Early builds of Windows 8 have been released without this blurring effect, and that could be why software rendering is enabled.
Here're the screenshots: