Several Android phones started showing Google Instant early, but Google's made the addition official, writing on its blog that a beta version has hit (or is hitting) "most iPhone and Android devices in the US". Basically, that's Androids running Froyo, and iOS 4 devices.
If you remember, this is when the search results actually start appearing while you're still typing out the search query. Some people have found it distracting and annoying, so rest assured you can switch it off on your phone just like you can on a computer. [Google]