The cause of this limitation seems to a power draw limitation in iOS 4.2.1. Before the update, the iPad could support compatible USB devices that drew up to 100ma of power. Skype headsets, most digital cameras and keyboards fell in to this category but after the update, power draw by these devices was limited to 20ma. When an incompatible device is plugged in, the following comes up “Accessory Unavailable – The attached accessory uses too much power.”
While iPad users won’t have their battery drained as fast, this creates a serious problem for photographers who want to import their shots while on the go – most cameras will draw more than 20ma of power. Even Apple’s own USB keyboard is no longer supported when using the camera connection kit. USB microphones are also affected along with any other odd device not directly supported but draws more power than the iPad will now allow.
However, it’s not know if this is a bug or implemented by Apple to manage the iPad’s battery. Apple has yet to address the issue and it is unknown if iOS 4.3 will correct this issue.