The "lo-fi" update should be used on anyone running any version of IPB 2.0.0. This fixes a problem where - depending on permissions for the forum - users could read topic content even when they are only allowed to view the topic titles.
The subscriptions manager update should be applied to anyone running ANY version of IPB 1 and IPB 2. The core module files will work for both versions. This fixes a potential vulnerability for users running MySQL 4.
The main download has already been updated so you do not need to use this update if you've downloaded IPB after this announcement.
Thanks to Tom for reporting the subscriptions manager issue in private