Google Now recently launched although it had one rather big restriction... it only works on Android 4.1 Jelly Bean devices. At the moment there are not too many devices that run 4.1 Jelly Bean which makes the app out of reach for many users of the Android platform.
Users over at XDA-Developers have been working on porting the app to older versions of Android though. At the moment they now have it working on Android 4.0 Ice Cream Sandwich. Although the team managed to port it quite quickly the voice features were not working. Good news is that the voice features are now working and Google Now fully works on Android Ice Cream Sandwich devices.
As with all customisations, mods etc... you need to install them carefully and follow all instructions. In other words, take the risk yourself if you wish but be aware that things might go wrong... read all the instructions first.
Full instructions can be found over on the XDA-Developers forums found over here.
For those not familiar with what Google Now is, take a look at the video below: