Hi, thanks for your post.
I just purchased a black 6300 optus model and have the same issue. Spoke to customer service at Optus, but of course the person answering the phone knows nothing about it (which is not their fault), thinks it's a Nokia support problem, transfers me to Nokia (Indian call centre). They take me through restoring factory settings on the phone, 'Optus Ambient' theme still defaults on power-up. I tried to delete the Optus theme using bluetooth software on my computer but it came up 'bad request' so the phone obviously won't let you do that. It seems like not such a big deal, but it is inconvenient to have to choose another theme every time you start up (and I'm not the type of user with a custom wallpaper picture, but many are). I think we should all try to get a response from Optus, either directly or through the store we purchased from, confirming that they have had the phones programmed this way. I think not only is it an unfair thing to impose on their customers, but it is also bad business. If I keep the phone, every time I start up I will have a bad feeling towards Optus and be less likely to stay with them in the long term. It's counter-productive to the free advertising they obviously wished to get from people looking at their 'yes' logo and the little gecko crawling around!