If I am understanding the situation correctly, we will be able to run Vista fine, but that the GPU may prevent multiple programs from accessing the 3D capabilities at once.
This may show up as a decrease in performance in either the Windows GUI (drawing the Aero images), or the performance of the additional programs attempting to access the GPU for the 3D functionality (i.e., games, video editing, and/or graphic intensive programs).
Again, that is what I got from the article as to what the average user may experience without the newer video adapters (which have not been released yet). Once Microsoft implements the new display driver model, and the hardware manufacturers make the video adapters available, we should be able to run multiple "video intensive" programs as well as the "Aero" shell without performance issues.
Unfortunately, this is not an easy "go down to the store a purchase a new video adapter (graphics card)" for notebook owners. 
