The Japanese word for a mobile phone or cell phone is keitai denwa ('portable phone'), written in Chinese characters. This is how the word is written in correct or formal prose, including the following:
In popular usage this is abbreviated to keitai ('portable'), as in the following advertisement:
As a Chinese-style compound read in the on-reading, keitai should by rights be written in Chinese characters. It is now much more common, however, to write it in katakana as keetai. This can be found both in informal writing and in advertising. Examples are easy to find.
Notice the very different environments in which keetai occurs.
Why the massive shift to? There are several elements involved.
1. The characters are relatively difficult and, more importantly, troublesome to write. The temptation to simplify is overwhelming.
2. Chinese-style compounds rely on Chinese characters to convey their meaning. In general there is psychological resistance to writing such compounds in anything but Chinese characters. However, by dropping the word denwa 'telephone' and passing into mass usage, keitai has become an informal, everyday spoken word, an identifiable phonetic shape in its own right independent of the characters that make it up. There is thus much less psychological resistance to a phonetic-style script (katakana) in place of characters.
3. Hiragana would be possible, but has two problems: unlike katakana, it fails to make the word keitai stand out from the surrounding text, and it looks childish. Katakana is the obvious choice.
The following are the results of a Google search in September 2003:
(Note, however, that 'mobile, portable' is found in a wide range of words, not merely in keitai denwa.)