You can use iphone explorer to change SMS tones; it gives you root access.
I suggest you back up your original tones.
FYI: SMS tones are aiff files with the .caf extension. You'll have to convert them if they're in mp3 format.
Are we talking JB or non-JB? Do you have a resource link? Thanks!
Until that time you'll either have to use Tri-Tone (the new sounds are totally unusable) or jailbreak your device.
Wow I didn't know that.. Apple is really restrictive on what we are allowed to do as far as customizing out phone.