Starting now, if you are running Froyo on your Android device go to google.com and look for a “Turn On” link below the search box from your mobile browser. If you are running Froyo and don’t see the turn on link Kanfesky says to refresh and it should be there.
Like “Google Instant” on the Google website, the mobile version also allows you to start typing a search term and it instantly matches real search results while you type. Naturally Google suggests you be on wi-fi, 3G or 4G while attempting to do this to get the results, well instantly.
“Google Instant” for mobile is still in Beta form so it may have some snafus. Although, with Google it could be in Beta form for years just like GMail. Kanefsky didn’t reveal when other devices running older versions of Android would see this functionality however we don’t think it’s going to take that long.
source: official Google Mobile Blog