I got an explanation from another forum, here it is:
"
AFAIK, you can't mod that message because it's not stored in a plist somewhere. It's a FlashSMS sent by SoftBank every time a new message arrives in your inbox. In the UK it's possible to send your own FlashSMS messages because the the code to do so on O2's network is publicly available. I've tried to find out SoftBank's code, but haven't had any luck. There's a thread about it at MacForums, which is where I got the attached image from.
As for Apple's "boneheaded" approach to new message notifications (or lack thereof!) I agree with you completely. It may be of interest to you that Apple has patented a lock screen notification system which would likely take care of this problem, although I don't know if/when it will be implemented. Apple also has a new iPhone/iPod Touch Design team working on UI stuff (the current icons were all designed by the Mac OS X team, BTW) and the fact that we haven't seen anything new from that team leads me to believe that Apple has some UI stuff up their sleeve for 3.0's launch this summer, much like they're keeping "Marble" closely guarded prior to Snow Leopard's launch. Maybe the lock screen stuff is on its way! I hope so, anyway."
On my phone the text in white is in Japanese.
I guess I just have to live with it.