There are a few ways of having drupal talk to FileMaker Server that we’ve used in the past before. Our demo registration forms interact with FileMaker via the WebForm and FileMaker form modules. This works great for this one particular situation of filling in a form and having that data sent to FileMaker Server.
But for more complex interactions, the best approach is going to be a mix of Web Service operations. There is a great post and webinar up on the MediaCurrent website about how they integrated Drupal with FileMaker using our very own RESTfm.
This is exactly the sorts of things we originally had planned for RESTfm, so it’s good to see it getting some traction out in the wider world.