http://img.skitch.com/20091025-ctfy4un9c9gyxq24gm97tf98er.jpg
Above it's a scan from a Hong Kong magazine. Those words I highlighted are "Cantonese-only" characters. Usually, if you see a small "mouth" 口 in the left hand side of a character, that is a Cantonese-only character.
In ancient times, Cantonese scholars borrowed wordings from Chinese literatures. However, since many people were illiterate and written Cantonese wasn't acceptable by the educated, most people only remembered the pronunciations and no longer know how to write those borrowed wordings. Later, writers of Cantonese opera, Cantonese versions of Buddhist text and Bible, created their own sets of "Cantonese only characters". (Religious texts and opera wrote in Cantonese for wider Cantonese audiences) Nowadays, some Cantonese scholars are trying to recover how ancient Cantonese wrote those "Cantonese -only" words and they are with some success. It turns out that "Cantonese only" characters aren't really Cantonese-only as most of them were borrowed from ancient literatures which other "dialects" in China and the Japanese also borrowed from.
The writing system of written Cantonese is same as standard written Chinese - both use Han (Chinese) characters. However, as you know, due to political reasons, the Communist party created a simplified version of traditional Han characters.
Written Cantonese is incomprehensible for non Cantonese. Firstly, because of Cantonese-only characters. Secondly, Cantonese has grammatical differences with Mandarin. Cantonese grammar only shares 60% similarity with Mandarin. Not many Hong Kong Cantonese speaker acknowledge with it and therefore they write strange standard written Chinese.