Windows to Linux Migration in the Enterprise?

youngerpants asks: "There is a lot of talk at the moment about migrating applications from WIN32 to Linux. This certainly helps move the OSS movement along, however, the true test of Linux is in the enterprise. Whereas we can move applications, how can the enterprise itself (such as Active Directory to Open LDAP, Exchange Server to Sendmail and NTFS to Samba) be moved. Have Slashdot readers used any applications or followed any strategies to migrate their enterprise? How would you tackle an obviously risky migration?"