I've now run into this issue a few times where after deploying Office Communications Server 2007 Enterprise Edition that the Office Communicator client is unable to download the address book from the IIS server. Below are some troubleshooting steps that I used to determine the cause and steps you can try to resolve this issue.
When starting Office Communicator, it will sign the user in but then present you with the following prompt to download the address book. Normally there is no prompt and Communicator downloads the address book seamlessly. This prompt will not accept any credentials so you must click cancel to stop the address book download.
After clicking cancel you then see in Office Communicator that the address book failed to download.
Let's begin troubleshooting this issue by discussing what is required to download the address book. When Office Communicator signs in it receives a referral from the OCS 2007 frontend to the address book URL. The default is a HTTPS URL which requires a valid certificate assigned to IIS. You can see the address book URL in the status page of the Front End server: