I haven't tested, but I'm fairly sure that applications can still access the accelerometer when mimimized in Harmattan, just the same as in Fremantle. There is no difference in multitasking capability. User experience may be different, because suspending UI updates when minimized is more common in applications running on Harmattan, partly because detecting and responding to state changes is more convenient in QML property bindings, and partly because some Fremantle software is written for desktop machines.