From a family point of view, adding user-account walls around these items would help parents control which applications their children can access. It also could create unique top score files, especially when said children come to you saying "I think I accidentally deleted the game save file you've been working on for the last month." Reducing the clutter in each person's home screens, presenting only those apps that each user prefers -- this would allow them to organize those apps on a user-by-user basis.
It's not just about parental control though. As Mike Rose wrote a few weeks ago, sharing your iPad with your spouse raises its own issues of comity and cooperation. With user accounts, you wouldn't have to make the hard decisions like "who is the primary user?" Each person could adjust their own application and system settings as desired. Couples wouldn't have to manage expectations about personal data and privacy so explicitly. And you wouldn't have to put up with all those Barry Manilow albums cluttering your perfect acid-washed rock library.
The jailbreak community has, as it often does, moved beyond Apple's current limits. iPhone developer "EvilPenguin" has been working on a way to introduce user accounts onto jailbroken iPhones. The technology remains preliminary; creating a serious account manager can be quite complex.
Many of Apple's iPhone innovations can find an early expression in the iPhone jailbreak world, which often pushes the iPhone OS into new and exciting directions. From voice memo recorders to book readers to iPhone location software, jailbreak technologies often find their way into official releases. Other jailbreak-originated innovations that have now made it into official iPhone OS firmware include copy and paste, spell checking, application folders, rotation inhibition, and multitasking, many of which were developed by Ryan Petrich of Boolean Magic.
Separate user account support remains in its infancy, even in the jailbreak world. The majority of iPad users, those who do not jailbreak, will have to keep waiting. Apple has not announced user account support for its upcoming 4.0 OS release. As Steve would probably say, "Just buy another iPad. Or a Mac. Not a big deal."
Thanks, chpwn, emock, AlJaMa, Optimo, DB42