One thing i noticed about GT3 and shimmering is that it depends a lot on what TV u're using. On certain TVs it looks absolutely horrible, on some others (mainly 100Hz or pro-scan TVs that tweak the interlaced image to make it more stable) it looks deecnt enough not to care. And the funny thing is that the shimmering on GT3 and many other games is MUCH more evident on Sony TVs, especially the ones they use in the shops blue booth thing. You know which ones...
Guess it will be the same for GT4.