I respect the "walled garden" as a business decision; it's Apple's platform, and maximizing its revenue is their prerogative. As a technical safeguard, there's no evidence that it works.
MobileSafari, MobileMail, MobilePhone, all running in the background. fairplayd is the fairplay daemon, commcenter manages communications (all of them), springboard is the desktop, lockdownd is the little thing that checks to make sure the phone is activated (and locks it down if not)
only the top three apps (bash, top, sshd) aren't there on a vanilla iPhone. Apple's apps will run in the background, but other (app store) apps exit completely when you return to the home screen.
Apple could easily write a specialized API that lets you run virtualized background processes. This would be an improvement on the current situation, but it would maintain heightened security.
With so much personal information on the mobile phone, it'll be a complete nightmare if one of the applications is really a virus that spreads as a background process. Since it can contact your ENTIRE contact list, such a viral infection can crippling. In fact, in the Jailbroken iPhone community there have already been a couple of viruses.
But on the other hand, they should open up a couple more APIs, the camera, specifically.