Apple wasn't able to use the iSlate name since HP already has a device named Slate out.
Even someone in Japan claims to own the iPad name, but i suspect this will go nowhere. So they came up with iPad, which to me is confusingly similar to iPod -- but maybe that is what they wanted to reinforce the fact that it uses the same OS programming as the iPod Touch and iPhone (whichis great in my opinion). That means millions of people already know how to use the iPad.