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What's cool about Facetime? It makes video calls using email addresses, which is pretty cool from a user point of view. It's not like there's a cellphone number attached to your iPad or iPad touch or MacBook Air. I'm guessing that from the email address it figures out an IP address to do the actual work, but like all of Apple's technology, you're not encouraged to peek under the hood.
Skype is available for the iPhone, but not available for the iPad yet. I expect it will be available by year's end. As an alternative you could use Fring, which also works on Android and Windows.