Mac Rumors noted that Dropbox for the iPhone and iPod touch now supports iOS 4’s fast app switching feature that Apple touts as a multitasking option. iOS 4’s other multitasking features include background audio, Voice over IP, background location, local notifications, task finishing, and push notifications.
The multitasking features in iOS 4 require an iPhone 3GS, iPhone 4, or iPod touch. The iPhone 3G supports a limited iOS 4 feature set, and Apple isn’t supporting the new OS on the original iPhone.