It would be really nice if we could have a reading pane in the conversation view. It's a hassle to have to double-click to open each conversation and when you open a conversation, you can't see other messages in the folder.
Yes, it's great to be able to group mail by conversations, but 90% of the time, when you're going through your mail, you don't care about the previous messages in each conversation...your focus is usually on the most recent message and it's more important to be able to see other messages in the folder, rather than other messages in the conversation.
Therefore, I propose that you give the user the option of displaying a reading pane in conversation view. If the user single-clicks on a conversation, the reading pane would show the most recent message in the conversation. If the user double-clicks on a conversation, then it would open the full conversation view, the same way as it does now.