I don't spend enough time writing about SKYPE, what I call the ICQ for VoIP.
Muniwireless.com has a good summary of what having SKYPE on a PDA can mean. I've played with other VoIP clients on my HP iPaq and, even with the 400 mhz processor, the voice quality still is lacking.
Until more powerful PDA's come along, these tools are more hobbyist in nature than carrier grade. But, in the future, and I suspect, the not too distant one, we will be using PDA's in lieu of a cell phone in some locations, and, for an enterprise or campus network, it sure will be a way to cut down the cell phone bill.