there are certainly keyword results that are a) in autocomplete, b) substrings of keyword results also in autocomplete, c) easier to rank for than the longer phrase.
Ask it another way: do you think a service that shows what the current google autocompletions are for every prefix of a given term becomes more or less interesting to SEOs after this announcement?
HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA
I clicked on it because it was so much funnier than the rest of them.
Turns out: affiliate link.
Or some sort of americanized japanese dating porn or something ... couldn't really tell.
Still. I definitely agree that it completely changes the SEO game.
Google auto-completes brands and keywords that only have 2 or 3 website references (I know of a site in stealth and I just tried it, and it worked. Google has indexed their homepage and title, and as I typed it in it auto-completed their name, with only a single website to reference).
If you have that much power, you probably have a better way to spend it. Also I would bet that suggest keywords come from user searches themeselves, not the web. That's why I suspect the impact of this is probably relatively minimal.
Call me crazy, but I think Google just might have considered the SEO impact before deploying this. If it's not literally on a checklist somewhere for deployment of search features, I'd be stunned.