I have a CBA Visa Gold (annual fee is $114), and hadn't bothered upgrading to platinum when it was introduced. Last week however CBA sent me a letter 'inviting' me to go platinum, with my annual fee not being due until my usual fee date. As this is not until March 06, I figured it was worth it so have signed up. I'm not sure if all Gold Card holders received this offer - my credit limit is $28k and I have been giving the card a bit of a workout lately, so maybe you need to be over a certain threshold. Anyway, my point is that they are offering a good deal to entice new members, so you'd hope that something similar would be on offer to keep existing ones.
Also, thanks to those that have posted advice about negotiating with Diners. When I signed up for the CBA Platinum I decided to chop my Diners, however after they assured me that I'm a very important customer, I encouraged them to make me an offer. I have now decided to keep it, had my points doubled for 3 months and my $95 annual fee from March reinstated.