We're actively working on a way to bring existing infrastructure under Terraform management without having to recreate it from scratch. The process will actually be really easy (but is vaporware at the moment): Terraform only needs the TYPE of a resource and ID of that resource. From there, it can "refresh" the rest of the metadata in.
Our idea is that you'll be able to say "I have an `aws_instance`, it's ID is `i-1234567`, and it satisfies the 'foo' resource" and it'll attach it.
Point being: we're thinking about this, and it is an important aspect of Terraform.
Another point is that you don't need to convert 100% of your infrastructure to Terraform to extract value from it. You can start by putting only specific services under management, and grow from there.