FWIW, I tried 6 apps that track and they really sucked my i4 battery quickly. I finally trialed and then bought MobileMe off of amazon a few weeks ago for $45 (via a reputable 'other seller') for all of our family iPhones and ipod touch devices (on an individual plan) and love it. No battery impact and incredibly precise. The other MoMe functions are a bonus.
So far, we've actually used it most frequently to locate the iTouch devices via the sound alert function in our own house - thanks to our 3 year old. Oy.