Communicator 2007 Automatic Sign-in Failure

In my case this was caused by lack of DNS entries.
Just creating a sip CNAME to my OCS 2007 server was enough, but there are others you can create.

I found a tool that helps you troubleshoot DNS issues here.

A\CNAME records:
  sip.domain.com
  sipinternal.domain.com
  sipexternal.domain.com

SRV Records:
  _sipinternaltls._tcp.domain,com
  _sipinternal._tcp.domain.com
  _sip._tls.domain.com

Communicator 2007 Fails to download corporate address book

In my case, this was caused by https certificate problems. The certificate had been generated during the install, but had not been applied to the IIS server.
Can also be caused by firewall issues with ports or IIS MIME issues for dabs (application\dabs) or lsabs (application\lsabs).

OCS 2007 Activation Problem

Failure to activate OCS 2007 during installation.
This can be based by having a space ( ) or quote (") character in the service account passwords.
Looks like the setup program does not quote the password correctly, so fails when either of these two charaters are used.