If you live in San Francisco this may look like good news. After EarthLink and Google will complete their project to offer WiFi throughout the entire San Francisco city ( http://news.zdnet.com/2100-1035_22-6058432.html ) you’ll be able to cut the costs associated with mobile phones by switching completely to VoIP.
In the same way you are now carrying your mobile phone with you in a pocket you can put a nice VoIP handset. The cost advantages may be tremendous especially if you are doing a lot of international phone calls.
And the market of small VoIP hardware and software phones is on the rise. If you are carrying around all day long a PDA then you’ll carry also a phone. Both Skype and SIP based services run happily on any WiFi enabled PDA.
If you prefer a hardware phone you should take a look at latest models coming from Linksys and DLink.
For a more complete offer you can always take a look in your favorite VoIP supply online store.