I currently use Lion on one of my machines and still use the beta version of iMessages. As any beta, it has its bag of issues but overall, it’s very stable. That being said, I ran into a situation this morning in which one of my conversations was constantly showing an unread message. Naturally, I tried restarting the app in hopes that this would fix this issue only to discover that I could no longer see my conversations window or start a new conversation at all. It’s as if the program was stuck. Restarting multiple times and re-installing did not help.
Long story short, the simple solution was to launch Activity Monitor and kill a process called imagent. As soon as I did that, all my previous conversations re-appeared and everything was back in order.