Perhaps the 4.0 software wasn't ready yet and so Apple decided to show 3.2 running on the iPad. Then, some time in March just before the release of the iPad, they will anounce OS 4.0 WITH multitasking. The new SDK is only released so that developers can start working on apps for the iPad.
Just a theory. Feeding from the thought that I just can't believe Apple would ship this device without the ability to multitask.