Google Instant should be useful on a mobile phone, especially if it doesn't have a hardware keyboard. When typing popular queries, it's amazing to see that Google predicts your searches from the first few letters.
Unfortunately, the mobile version of Google Instant doesn't have all the benefits of the desktop version. Since the virtual keyboard covers most of the screen, you can't see the search results until you hide the keyboard. Another issue is that you can't press Tab to use Google's prediction and continue to type your query. Google Instant doesn't work well if you use a mobile Internet connection because of the high latency.
Google says that you should notice "a big improvement in speed when you search thanks to a new AJAX and HTML5 implementation for mobile that dynamically updates the page with new results and eliminates the need to load a new page for each query. Google Instant for mobile works best on 3G and WiFi networks, but since the quality of any wireless connection can fluctuate, we've made it easy to enable or disable Google Instant without ever leaving the page".