Can I use WiFi to make free calls (as with skype) using a web-enabled PDA or smart phone?

Seems like a no-brainer for the hacker community... Hack the web-enabled PDA or smart-phone to use free VOIP while at free hotspots so you're not burning up minutes. I guess a snag might be if the PDA (like a Palm TX) doesn't have a mic. Anyone know 'bout this? Got a quick link to a Forum Thread showing how to do it??? Thnx!