The latest build of the iPhone SDK (beta 4) and accompanying iPhone OS 2.0 beta release (5A258f) include support for "active" and "inactive" applications. Though some developers initially interpreted this inclusion as the addition of the ability to run third-party applications in the background -- a notable omission from the initial SDK. However, it appears that the new capability only allows third-party applications to remain active while the device is locked (asleep) or there is an overlaid window.
Apple's documentation reportedly states: "When an application is inactive, it is executing but is not dispatching incoming events. This occurs when an overlay window pops up or when the device is locked."
Regarding the use of background processes, Apple's documentation still says:
"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 are using quits."