A few suggestions that I decided to post on this site as well. Conversation IS a good program, with a lot of potential, but these are just some of the improvements that I believe if made, would make it even better;
1. The way it's a phone book extension. It has to do with text messaging, shouldn't it be part of messaging?
2. When you receive a new text message and you're viewing it through Conversation, you HAVE to open it to remove the new text message icon (mail/envelope logo located on top right), even if the message is just a simple one-worded one, like 'Okay'. This one's a bit hard to explain as to why it might be somewhat important.
3. The ability to see the whole message - without having to open them. To view long text messages, you have to either highlight it and wait for it to scroll, or open it. To me, those just kind of defeat the purpose of having an application like Conversation-- whose sole purpose is to thread SMS messages to make it easier to read + keep track.
Instead, it should just list it somewhat like a chat window, something that you can easily read and type back.