This would not be possible for an iphone.
I'll look further into the idea for android OS, though.'
edit: The same problem would exist for android phones, as well. You can change the .png of your current smileys in the FW, but only YOU would be able to see the change. The person you send it to will just get
, unless they also replaced the .png files. The only way to make these smileys work properly, is by sending them as images (MMS).