iPhone app that lets you put nearby Macs to sleep (discovery can be handled via Bonjour). Unlike iShutdown or Control!, it shouldn't require installing a special daemon on the Mac that's to be controlled. iSleep claims to do this via Remote Login (i.e. ssh), but the reviews seem mixed. If remote Apple events are still possible, then it should be feasible via that. If not (or as an alternative), whatever mechanism Apple Remote Desktop uses could be used here too.