I bought ME1 from retail (years ago), before it was released on Steam, so it's currently not linked to my Steam account. Is it possible to activate it yet?
you can't activate it on steam, but if you install from the CD/dvd you can create a shortcut in steam to it. This will let you launch through steam, steam will update your status with what you're playing and enable the steam overlay UI while playing it as well. nearly all the benefits. you'll still need to have the CD in while playing and steam will not auto-patch it.