This is a feature I have wanted for a really long time; to be able to find an email in your Inbox and copy and paste a link to anyone without having to find the message in the online archive is going to save a lot of time and decrease some latency when chatting on IRC or some other form of real time communication.
OK, how do I easily get this information out of the header and use it without having to view the email source every time?
I use Thunderbird for most of my email viewing (sometimes mutt for composing). In Thunderbird there is the Archived-At plugin. After installing this plugin you can right click on any message from a capable mailing list and select "Copy Archived-At URL".
Now you are off to share that URL with friends :)