ILM 2007 Feature Pack 1 is actually a suite of two products, an updated version of Microsoft Identity Integration Server (MIIS) and Certificate Lifecycle Manager (CLM), previously idNexus which Microsoft obtained after acquiring Alacris.

MIIS is probably most famous as a tool to assist in Cross-Forest Exchange topologies (two separate exchange instances in their own forest glued together). MIIS is then used to synchronize the Exchange Global Address List (GAL), which enables a consistent addressbooks, mail routing and sharing a SMTP namespace between Exchange organizations.

CLM is the Microsoft product to manage the lifecycle of (x509) Certificates and Smartcards.

MIIS 2003, ILM 2007 and ILM 2007 FP1 will cost you money.
But Identity Integration Server for Microsoft Active Directory (SP2) (IIFP) is FREE and can be downloaded here.

This is a lightweight version of MIIS 2003 which can only be used with Active Directory but can be used to setup GAL synchronisation.
There is catch with Exchange 2007; the ILM 2007 version will run the powershell cmdlet update-recipient automatically for you.  IIFP won’t do this, so you’ll have to setup this yourselves ,which is not a big deal.

A new version of ILM is underway and for now called “ILM 2″.

More details.

Technet July 2009 – Managing Active Directory users with ILM 2007

“ILM 2″ Product Page

Introducing Certificate Lifecyclemanager

ILM 2007 FP1 Product Page

How to deploy Exchange 2007 in a cros-forest topology