I don't under stand, an app for you iphone that works like mobile me? It won't exist because the mobile me one kinda hi-jacks the phone in a way. Forces the iphone to send gps data in the background. For an app to do the same thing, the app would have to be open and running so every time your done with your iphone you would have to open the app back up again, because apple doesn't allow apps to run in the background and I don't know of any app like this in the jail break scene either.
if you jailbreak, ilocalis does this, it's a pay service though. And I should say, it's not polished at all, in fact, pretty ugly.
Why don't you like MobileMe? IMO, anyone without MobileMe that has an iPhone is asking to lose their phone. It does it VERY well, and has some amazing other features I can't live without.