Apple is really taking baby steps...keep in mind that it's only a small portion of apps that are affected by this restriction. Games or utilities can still save their application status to disk often allowing you to resume where you left off when you start it back up.

But the single biggest issue we’ve found is in the 100 page iPhone Human Interface Guidelines. It’s a public document, but you must be a registered iPhone developer to see it. We’ve embedded it below via docstoc.

Users can only run one application at a time, and if they leave an application it quits. That doesn’t seem like a big deal, but it means that you can’t switch away from an application and have it continue to do things. That’s a big issue with the current support for websites on the iPhone - as soon as you leave the browser the connection is broken. With the iPhone, the hope was that an installed application could continue to run in the background and, most usefully, gather and send information from and to the web.

Only one iPhone application can run at a time, and third-party applications never run in the background. This means that when users switch to another application, answer the phone, or check their email, the application they were using quits. (p. 16)

[Via TechCrunch]