It is clear that the number of fine-grained IT authorization policies that are spread across a medium-size or large-size IT environment can easily go into the 10,000's and 100,000's. Just take the rules from firewalls, databases, and single sign-on systems, and you see that the complexity has grown out of hand: Security is simply unmanageable.
Today's authorization management solutions (sometimes called "entitlement management") tackle the problem by simply putting all the complexity into a single place (the Policy Access Point, PAP). By and large the rules in the central manager are still at the same semantic level and complexity as the rules that are spread across the IT environment if no authorization management is used. This is clearly not a significant reduction of complexity.
(By the way, identity management does not actually cover this problem very well, as it is pretty much concerned with managing identities and less with the management of fine-grained, expressive, maybe context-sensitive authorization policies).
In summary, today's authorization management makes the problem evident, rather than solving it.
What today's vendors are good at is solving the policy interoperability challenge: XACML is a webservice standard for exchanging authorization policy information, and vendors include ObjectSecurity, Cisco, CA, etc.
Model driven security is concerned with solving the complexity challenge: It lets you manage simple, business-driven security policies, and generates the 100,000's of rules for the particular deployment automatically. Sounds like magic, but it is not. Contact ObjectSecurity, the leading model driven security vendor if you would like to learn more.
So in summary: authorization management is necessary but not sufficient.