Using morse code, perhaps? With only one button, there isn't really any other way to write (I presume that if you were using speech recognition, you would have said "saying" or "speaking," not "writing").
(Yes, I know that you meant you're using a touchscreen on an iPhone, or something similar. I'm trying to be vaguely humorous, and probably failing.)
(The 3GS, according to wikipedia, has 5 physical buttons and then the touchscreen. The iPhone 4, from the pictures I've seen, has at least 4, and the touchscreen. The fact that people entirely forget about them suggests that the design is extremely good, if people are using them without realizing it, or quite bad, if people aren't using them or realizing that they exist. I am inclined to lean towards the former.)
(Yes, I know that you meant you're using a touchscreen on an iPhone, or something similar. I'm trying to be vaguely humorous, and probably failing.)
(The 3GS, according to wikipedia, has 5 physical buttons and then the touchscreen. The iPhone 4, from the pictures I've seen, has at least 4, and the touchscreen. The fact that people entirely forget about them suggests that the design is extremely good, if people are using them without realizing it, or quite bad, if people aren't using them or realizing that they exist. I am inclined to lean towards the former.)