Can I make VoIP calls on the iPad?

All signs point to yes.

The iPad SDK (Software Development Kit) for iPhone OS 3.2 allows developers to make VoIP (Voice over Internet Protocol) calling applications. The iPad has an integrated microphone and speaker that should work for voice calling over the Internet.

VoIP apps have already been developed for the iPhone. The iPad SDK is novel in that it allows VoIP calls to be made over the 3G cellular network. Apple and AT&T had previously limited VoIP calling to Wi-Fi only.

This means that on any model iPad making voice calls over the Internet will be possible!