I think it's a great idea to (1) support multi-user functionality, including end-user customization and parental controls, (2) provide parents more control over purchases on their iTunes account, (3) improve corporate security, and (4) simplify password entry for applications and web sites.
Wirh multi-user functionality, each user would potentially have their own profile tied to a scan of their fingerprint. Each user would be able to customize the device with their own background, icon layout, etc. In settings, the master user could designate which users (=fingerprints) have access to which apps, and users would only see their authorized apps. The feature could also be used in combination with parental controls; the master user could enable parental controls (limits on usage per day, permitted time of day, filters for questionable content, etc.) for other users.