Many probably know that Lotus Symphony is based on Lotus Expeditor and Eclipse but you may not have put two and two together for new opportunities. You can install pretty much any Eclipse plugin into stand alone Symphony; by default the Eclipse plugin install menu is enabled:
Now, in Symphony 1.3, there is also support for composite applications. So if a Notes or Expeditor developer create a .CA file a Lotus Symphony user can open it. Of course you will have to make sure any features developed for are linked in the .CA file so it can be launched correctly.
So from a business partner point you can provide plugins and composite applications for the Symphony product suite. You can also deploy entire RCP based applications to Symphony. If you have the coolest plugin you can essentially deploy it to Symphony, Notes, Sametime, and Expeditor installations.