iPhone app that can send emoticons to non-iPhone devices
Emoji, as the iOS emoticons are called, are Unicode characters that need a certain set of glyphs to be able to be displayed—think of it as a missing font not displaying correctly. If the device doesn't have the "font", it can't show it.

What you're asking for is impossible: you can't do that without a custom font installed on both devices, and on most smartphones that's not possible to do. 

However, if the person on the intended receiving end is using a Mac running OS 10.7, they will display, as Apple decided to make them available in Lion.