Developers have found further evidence of multitasking support for 3rd party iPhone apps in the latest beta (3.2 beta 4) of the iPhone SDK, and suggest that it will become a reality this summer.
9to5 Mac reports on a new line found deep within the latest iPhone SDK. Specifically, SpringBoard.js has a reference to a "multitasking dialog box" that did not appear in version 3.1.3 of the SDK; it seems that it's new to version 3.2. Of course, there's no assurance that this refers to 3rd party support for multitasking, but it is new.
Additionally, Appleinsider's souces with "proven track records" state that Apple has developed "a full-on solution" for 3rd-party multitasking which will be a part of iPhone OS 4.0. No specifics were given on how it will be pulled off or how it will address the two main concerns: battery life and security.
Let's assume that Apple's plan addresses the security issue, but battery life still presents a problem, one that was supposedly addressed by Push Notifications. Apple's remote notification service allows applications to offload polling processes to web servers. By keeping the update algorithms working off the device, the iPhone's battery is spared. Certainly the iPhone itself must take on the task of keeping all of those apps up and running.
It should also be noted that iPhone OS does not use a paged memory model. That means, multi-tasking applications must compete for the same memory space, making it more likely that apps will receive memory warnings and even crash when they use too much memory. That's not an issue in the one-app-at-a-time space, but a real problem with multitasking
Of course, the iPhone OS is already fully capable of multitasking. In order for non-Apple apps to participate, Apple must lift the current restrictions within the OS. That's something the company won't do until the iPhone engineers have devised the best and safest method. As for iPhone OS 4.0, Appleinsider notes that it's got "a ways to go." Hopefully we'll have an answer in July.