I'm not clear on how Google Now would even work on an iPhone. I can see how it would function when you physically launch the app, but that kind of defeats the beauty of Google Now -- helping you throughout your day without having to tell it to do anything at all. It is phenomenally useful (almost scary useful) on a modern android device, but I couldn't imagine being able to fully appreciate it if it only worked when I hit a button telling it to launch.
Actually, it'd probably work fine.
Contrary to popular belief, iOS has multitasking features.
You just need to meet certain criteria. One of which is what some people find really cool or really creepy: location/navigation based features.
From then on, it's allowed to run in the background and update yourself on your current location, and issue notifications to the user.
Depending on how often it uses the location data and radios, you might be hosed when it comes to battery life though.