Skype is breaking new ground by trying to bring PDAs that can connect to the Internet into the VoIP mix, said Zennstrom, who co-authored the software the Kazaa file-sharing network uses. "It was one of the top requests from our users," he added.

But the push to PDAs could have drawbacks. To make VoIP calls, a PDA would need to have a broadband connection. For now, only a limited number of PDAs that have built-in Wi-Fi connections would be capable of making Skype calls.
Free Net calling goes beyond the PC | CNET News.com

There are a few issues when using VoIP on a PDA. (1), PDAs need a Wi-Fi connection (2), Network Address Translation (NAT) traversal is often problematic. (3), Wi-Fi VoIP Roaming needs special support.

Still, VoIP vendors are racing ahead with solutions. The Proxim AP-4000 Access Point (MSRP $899, $650 street), for example, is designed for roaming VoIP and supports pre-802.11e, the standard designed for quality of service (QoS).

Competing VoIP software for wireless PDAs is available from Palm Computer, Pocket Presence, Telesym and VLI.
Daily Wireless - Mobilizing VoIP