To withhold your number when sending a txt is to write your message as if you are putting a name in the phone book but save it without a number then send it as a Business card.
Another way: You need a phone with reminders on. Write what you want and send it but remember that the person you've sent reminde has to have reminders feauture in his/her phone, otherwise your number will show up. Most of new Nokia and Sony Ericsson phones have reminders.