[SonicMouse]

I figured you could add more SMS tones to the iPhone, but you can only replace the 6 that already exist.

In this directory:
/System/Library/Audio/UISounds/

There are six tones:

1. sms-received1.caf
2. sms-received2.caf
3. sms-received3.caf
4. sms-received4.caf
5. sms-received5.caf
6. sms-received6.caf

So, to replace them...

Make sure you have an SSH server on your phone (install OpenSSH from Installer app)

Open iRinger, create the sound you want

In iRinger, click Export, uncheck the "export to iTunes" checkbox
Select "no gap" in the looping box
For Ringtone Title put: sms-received<n> (<n> = a number 1 to 6)
Check the "Export via SCP"

in the SSH dialog:

Enter the details to your phone's SSH server then for the Target put:
/System/Library/Audio/UISounds/%TITLE%.caf

Hit "Go"... and you will have the custom SMS tone.