Today I found a feature in Lotus Sametime 7.5 I was not aware of. If you reply in a chat window after your buddy went offline, you will get this dialog box:
While I particulary like the Skype feature to continue your chat even if your buddy is offline (GTalk does provide that feature now too) and send the messages again in the background when your buddy comes online again, IBM decided to go the other way. Not really bad. So you can change from synchronoues to asynchronous communication by changing the protocol. It will take the chat transcript, put in a new email adressed to your buddy and you can add your reply into the email body:
Personaly I like more the Skype way to keep all messages in one context. Maybe an option to choose between both ways would be great.
Update: This feature only seems to work if you use Notes 8 with the embedded Sametime 7.5 plugin.