Apple's Find my iPhone is a universal binary that's optimized for both iPhone and iPad. Looking at Apple's other App Store apps, as indicated by the + icon, Find my iPhone is also the only Apple app that's a universal binary. Apple's iPhone apps, Apple Store (released just a few days ago), MobileMe Gallery, MobileMe iDisk, Keynote Remote, Remote (for Apple TV/iTunes), Texas Hold'em will all run on the iPad in 1x or 2x mode, but they don't look great. Apple's iPad apps, Numbers, Keynote, Pages, and iBooks won't run on iPhone.
Apple has announced iBooks is coming to iPhone with iOS 4 but it hasn't been released yet. Neither has iOS 4. That's coming Monday, June 21. That's probably when iBooks is coming as well, by way of popup notification when you launch the new iOS for the first time -- just like with iPad.
But what about those aforementioned non-universal binary apps? Lots of you want to use an optimized version of Remote to control your iTunes and Apple TV on the iPad, or MobileMe Gallery to access your online images, or iDisk to get at your files and media.You keep telling TiPb as much.
Maybe they're coming with iOS 4 as well. I just hope it isn't iOS 4 for iPad, due this fall. I'd like to use them between now and then...