The Conversations app. is my most used application on the phone. I have some suggestions:

* Conversation: Show the whole message instead only the very few words. It should be more like a chat. Take a look at the great interface of Windows Live for S60.

* Mark read: Why do I have to open each message to mark it read? If you implement he full message view, it should be marked read automattically when one brings up the conversation.

* Font type/size: To make full messages viewable the font is far to big. Please make more options available.

* Replace existing SMS app: can you make this app default?

* Show conversation app when new message arrives

* Hide reply-box until one starts typing to reply. This way we gain more space to view whole messages and much cleaner look.

* Skin support: I would love to sea the iphone bubble view.

Please take a look at the implementation of threaded sms conversations on the iphone, it very well done.
Example Image(Click)