Since the 3GS (and 3rd generation iPod Touch) has both more RAM and a faster processor, you don't see checkerboards much. You still see them a lot if you scroll while the page is loading.

An example, I just did: take a big, long, complicated page with lots of Javascript (say engadget.com, the regular non-mobile version) and wait until Safari is 100% done loading. You can scroll at skimming speed and I only saw the checkerboards once. But if you scroll like a madman trying to get to something near the bottom, it'll still checkerboard and make you wait for the roughly 500 milliseconds to render.

The 3GS does the same thing if the page is big (long?) enough and actually releases content so if you scroll back up it has to be rendered again.