I looked thru the Forums to see if anyone has attempted or tried the Apple Trackpad with their iPad. It comes as a Bluetooth device so I would assume it would work with the iPad.
As already mentioned what would be the point. Without an on screen cursor it would be pointless. Not to mention it would defeat the purpose of having a touch screen device.
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The iPad bluetooth stack has no support for mice or trackpads (or a lot of other BT devices). If you jailbreak there is an alternative Bluetooth stack that you can install and then install mouse support on top of that (don't know if trackpad would be supported). It also adds a cursor