On the TechNet page that explains your options for the STSADM Sync command, there is no explanation for the IgnoreIsActive parameter. This is what it does.
The IgnoreIsActive flag allows you to choose which profiles to sync. By default, only the active profiles in a content database get sync'ed. An active profile is one of a user who has contributed something to the site. If you want to sync all users in the site, you can choose to trigger the execution of the sync job via STSADM with the IgnoreIsActive flag set to 1, and it will sync every profile.
You can investigate what profiles are marked as active, by looking in the UserInfo table of a specific content database. The tp_IsActive field will have a 1 for active, 0 for inactive.
Great post. I have been searching for a few hours on this and finally saw the parameter of -IgnoreIsActive. Not much out there on this but this should be one of the first things you look at if your User Information List is not getting updated by the User Profile Service. I ran the stsadm -o sync -ignoreisactive 1 and it updated my user information list. Thanks for the post.