I have two comments about Google Instant:
1. How many people actually search from the main google.com window? If they use the search window in Firefox, Google Toolbar, or the address field in Chrome, Instant doesn't work for now. So a large segment of the population will never see Instant.
2. Instant has implications for long tail keyword searching done on the main google.com window. Because it is filling in data with a combination of the Wonder Wheel, Localization, and now personalization, the user has some incentive to stop typing and go after the Wonder Wheel term, for example. I tend to build sites around the Wonder Wheel already, but if you have gone into deep, non-Wonder Wheel long-tail keyword optimization, your traffic will be cut to the extent that Instant provides a sufficient result. And this would again only be true for those searches uttered from the main google.com window, because the other search windows are currently not affected.
It's not the end of the world for sure, but it will affect some adversely.