If having emoji icons on your iPhone keyboard is not enough, maybe you can follow this little trick that will easily allow you to add symbols to your text entries.
To do so, go to http://mrgan.com/gb/ from your iPhone and follow the onscreen instructions. It will add a bookmark to your homescreen from which you will then be able to copy/paste various symbols.
There are 48 symbols right now but more will probably be added in the future.