If you're itching to give Google Now a try, then, once again, xda-developers has you covered. You can now run Google's new assistant app on your Android 4.0 handset provided you're running a ROM based on the Android open-source project (AOSP). If you attempt to install Google Now on a non-AOSP ROM, you'll run into crashes and possible boot loops, so, please don't do it.
The method for installing Google Now is a little more complicated than that of a regular APK. To get it running, you must replace the APK for the Google "quick search box" with the new Google Now APK, and then set the correct permissions. As all of this takes place in the system folder, installing the app requires root access (which shouldn't be an issue if you're running an AOSP-based ROM anyway). Currently everything seems to be working as it should, aside from voice search, but don't be surprised if you come across instability or odd behavior — Google Now wasn't made to run on Android 4.0, after all.
The full instructions for installing the app are available over at xda-developers, along with the custom APK you'll need to get it working. As always, do not attempt to modify your system files unless you're sure of what you're doing. To reiterate, you must be running an AOSP-based ROM for the APK to work. A large number of custom ROMs are based on AOSP, including CyanogenMod, MIUI, and AOKP.