Rumors about multitasking on the iPhone have come and gone many times over the last couple of years. It's been one of the most asked for features ever since Apple started supporting third party apps with iPhone OS 2.0, and represents a major shortcoming when compared to other smartphones on the market. The hardware is clearly capable, as "jailbreakers" have proven, but Apple artificially limits this functionality to a few sanctioned apps due to battery, resource and security concerns.
According to AppleInsider, however, an upcoming iPhone 4.0 software will finally bring multitasking to the platform this summer. The "full-on solution" will reportedly support several third party apps running at once in a "multitasking manager that leverages interface technology already bundled with its Mac OS X operating system." This could potentially refer to the basic Command-Tab switcher, or something like Expose.
Unfortunately, no more details are available at this point. The change in policy is likely related to a future hardware update. Rumors suggest that the next iPhone -- likely to be announced around June -- could use a variant of the A4 processor found in the iPad as well as additional RAM.