For those who are interested; on the Dividing Line Broadcast, Dr. James White discusses the issue of why certain Reformed believers have signed this document.
The bottom line seems to be on how they understand the document to be understood. As Dr. Ligon Duncan (one of the Reformed signers) says “If I thought that the document compromised the Gospel in any way, I would not have signed the document.”

Yet, I believe that Dr. White made it very clear that the drafters (Chuck Colson & Timothy George) of the document actually consider Roman Catholics to be Christians. This is clear by Chuck Colson's comments about the document.
Similarly, if we are going to be able to properly understand Scripture, we are going to need to understand what the original authors of Scripture intended. In the same way, if we are going to understand a document such as this properly. We need to understand the authors understanding and intentions, or we will fall into error.
I hope I stated that coherently?

Anyway those are what I took of the Dividing Line Broadcast, which can be listened to here

Tom