Although Diablo III is not available for purchase in Steam, you can still add the Steam overlay so that you can chat to and share screenies with your Diablo III friends. Here is the simple way to get a working Steam overlay in Diablo III.
- Open up Steam
- Click on "Games > Add a Non-Steam Game to My Library".
- Browse to your Diablo III install folder and select "Diablo.exe" (not launcher).
- In your Steam library, right-click Diablo and select "Properties"...
- Add -launcher.exe to the end of the "target".
- Play Diablo III from your Steam library.
- SHIFT+TAB to access Steam in Diablo III.
Now you can interact with your Steam friends in Diablo 3.
The downside to this is that you wont be using the launcher, which provides the updates before launching the game so you will occasionally need to restart Diablo 3 when it notifies you that there is a new patch available.
Battlenet really needs to up their game with their Battlenet GamerTag so that all Battlenet gamers (WoW, StarCraft 2, Diablo III) can interact with each other.