It turns out that there are at least two ways you can build custom applications that take advantage of OCS 2007’s capabilities for presence, IM, conferencing, and voice. First, you can use the Communicator SDK to do client-side stuff, like asking Communicator to initiate a voice call or change your presence status in response to some action. (I need to find out if Communicator is smart enough to set your status to “away” when you lock your workstation; if not that will probably be my first project.)
Second, you can use the OCS SDK to write server-side code for things like IM content filters. This is also a handy way to write server-side response bots. You can also integrate presence and IM with SharePoint using a fairly simple web part; I’ll write more about that later.