the new abbreviation to learn . MSFT engineers this as compatibility for boxes with no video cards. Do really market requires it?. Beta shows satisfactory performance over intel integrated graphics.
Well, more details is at WARP. Interested thing is that Aero on Win 7 will not require any integrated graphics and adds that any CPU over 800 MHz will run.
Ironically, it requires 512 MB RAM and it will use GPU. They lament on Front Side Bus transmission speed i.e. 10 GBPS against 25-100 GBPS speed of video cards. However, MS has at least start considering performance on native hardware instead on relying additional for running basic operating system.
Below is sample image for rasterization.
(Source from http://www.cdrinfo.com/images/uploaded/Ray-tracedVsRasterized.jpg)
At least better than current renderings.