Sounds like this problem is still unresolved?
Here's a long shot. Are any of you guys using workplace email accounts, where emails are stored on a Microsoft Exchange Server? I'm seeing the exact same problem on a Lion mac at my office running Outlook for Mac 2011. It seems like a funny conincidence that two different mail programs could have the same weird bug, so maybe the bug is actually further upstream - something to do wth the communication between the operating system, network, and mail server?
Maybe the 'Sent mail' folder data is being loaded from the server in a way which doesn't flag up that it's a sent mail folder, which then causes various email clients (Apple Mail, Outlook for Mac) to get confused and treat it like a regular folder, listed by sender?
Just a thought. If this is the case, the fix won't be in Apple Mail (or, in my case, in Outlook), but will be in something further upstream. This would also explain why some of us have this all the time, but lots of people have never even heard of the issue.
If you guys could share any known details about the account types that have this problem and how the data is retrieved (and also, any acount types that don't have this problem) we might edge closer to finding a fix.