A flash SMS or class 0 SMS is a text message which the recipient can't save and which doesn't include your number.
1) Install minicom from cydia
2) Configure minicom (1st time only)
In terminal type this will open a graphical utility to configure minicom. Select "serial port setup"] and hit return. press and set serial device to and hit return. now press “esc” and in the main menu select “save setup as dfl”
hit return, and now select “exit”
Now hit ctrl+c and it should exit the minicom utility.
3) ssh into your iphone
7) Change the modem sms-format to PDU (1st time only)
AT+CMGS=(the number from the site)
> (enter the PDU-string from the site)
11) Your flash message is sent.
This was tested on an iPhone 3G. Any developer wanting to create an app for this is greatly encouraged. I am not responsible if you mess up your phone or something else.