Various monikers have been suggested for the yet-to-be-announced Apple tablet(s), with varying degrees of evidence in the form of domain names and trademarks already acquired by Apple or others acting on behalf of Apple. These include iPad, iTab, iTablet, iPod tablet, iBook, Macbook touch, and now iSlate and Magic Slate.
It's good that there's an assortment of names to choose from, as there are at least three potential products in need of one, including, besides the two I've already described, a touchscreen peripheral to be connected to (or integrated into) your laptop or desktop keyboard, as described here.
While you might see iBook touch and Macbook touch applied to versions of those laptop lines with integrated display touchpads, that would suggest an intention to continue to produce versions without display touchpads, which, if it happens at all, is practically certain to be a temporary state of affairs, and Apple would find itself in the position of having to either keep around or unceremoniously drop a designation meant to differentiate one model from another within the same line.
iTab just seems awkward, so I'll eliminate it from further consideration.
Approaching the question from the direction of specific devices to be named, and starting with what is presumably the smallest, we first have the i/o display touchpad, which may appear first as a peripheral then later be integrated. The integrated version doesn't necessarily need a name, as distinguished from a description, but the peripheral version certainly will. It might be called iPad, or iSlate, or Magic Slate, or Magic Pad. (Note that there is an iPhone app called Magic Pad.)
Then there's the smaller, more portable standalone tablet device. It also might be called iPad or iSlate, or iTablet, or maybe even iPod tablet.
Finally there's the larger, less portable semi-standalone tablet device. (I say semi-standalone because most of its use cases are sure to include continuous communication with some other device, and it's likely to need more frequent recharging than the smaller tablet device.) It too might be called iPad, iSlate, or iTablet, but probably not iPod tablet.
The peripheral and the smaller standalone tablet device might actually use the same screen, but the peripheral wouldn't have more of a radio than Bluetooth/Wifi, if that, and might not even have a battery. As with Apple keyboards, it could come in both wired and wireless versions.
My personal preference (not a prediction!) would be to call the peripheral device Magic Pad, the 7-inch tablet iPad, the 10-inch tablet iTablet, and reserve iSlate for an even larger, less portable device, on the order of those depicted in the newly released movie Avatar.