I've got a bit of a dilemma. I'm 3/4 of the way through RTK... but now I have a Cantonese speaking girlfriend, and intend to switch from learning Japanese (at least for the time being). I've kinda been carrying on through RTK regardless, on the assumption that the Japanese characters are basically the same as the traditional Chinese characters, but I figure it's probably best to check.

So, my question is, how different are kanji from the characters used in Cantonese? I notice that someone very kindly put up a cross-reference of simplified Hanzi with RTK, but I can't find anywhere a list of their similarity to kanji, nor even a list of the most common characters found in Cantonese (and believe me, it's not for want of trying). I was looking at Zhongwen, but I don't know how useful it'll be for Cantonese, nor whether it's worth all the hassle and cost of importing the book.

On a related topic, does anyone know of any good places to find Cantonese material online? Y'know, blogs, news sites, podcasts, other random stuff... that sorta thing. Many, many thanks in advance

Keep in mind that Cantonese speakers generally write in Standard Chinese, just like Mandarin speakers. There is written Cantonese, but it isn't standardized and it may be kind of tough to find much material for it. Hong Kong has a list of characters taught to schoolchildren called the 常用字字形表 (Soengjung Zi Zijing Biu), which contains 4759 characters. I'm not sure where to find the list, but maybe that will help.

So everytime you switch a girlfriend that speaks a different language you'd switch? :o
Well lot's of characters are the same and lots of them are not as in different usage, Japanese has their own simplified characters (although not as dramatic as Chinese simplified) and characters not used in (standard) Japanese. Adding to that, spoken Cantonese is very different from written, for example:

(spoken) 你食咗飯未呀？
(written) 你吃了飯嗎？
"Did you eat yet?"

(spoken) 乜嘢呀
(written) 什麼
"what"

(spoken) 佢
(written) 他
"he/she"

To give you an idea.
Written Cantonese is essentially the same as Mandarin, just using traditional characters. Though grammar can differ in certain areas.
It kinda depends what you want to do, only speaking, or just reading etc.

Ok, so it'll just be okay to go with the traditional characters... thanks very much really I want to focus on conversational Cantonese, but it'd be nice to be able to read and write it since I've already learned a lot of characters.

Also, I didn't learn Japanese for the sake of a lady, in case you were wondering I'm not some kind of inter-lingual hussy

"inter-lingual hussy", hehe

Anyways, good luck with it!
Just a FYI, if you thought Japanese particles are tough, Cantonese has 30+ particles,
and you can even chain multiple together! (*evil laugh)

Your new girlfriend probably can also speak mandarin, so I'd learn that as it is much more useful and there is more learning material out there.

Or you can learn Japanese together.