I have used the Talkatone app http://goo.gl/0N13B along with a Google Voice number on my iPad for making and receiving phone calls with a WiFi connection for over a year now, and it just simply works. To be honest with you, on the cool factor level this is 5 stars. First make sure you get yourself a Google Voice number that you will only be using for your iPad. If you already have a Google Voice number, but are using it, then get a new Google Account and a Google Voice number that you can use just for this purpose.
I always suggest getting a Gmail account that you will never plan on using along with a Google Voice number for that account. The only purpose here, is to get your Google Voice number.
Talkatone uses GoogleVoice for it's service to make phone calls (VOIP). The reason you need a Google Voice number just for your IOS device (This would work on an iTouch or iPhone too.) is because you will do some special settings in your Google Voice that will make it tough to use that same number for your cell phone or what ever you are using your Google Voice number for.
I am not going to reinvent the wheel with directions on how to set this up, so I will just give you a few links to get you started.
From TechCrunch: Talkatone Lets You Make Calls And SMS Over Wifi For Free, No Strings Attached - http://goo.gl/0ShHM
Make Free Phone Calls Over Wi-Fi/Data Using Talkatone [Android & iOS] http://goo.gl/EqKyf
Google Voice Home Page: https://www.google.com/voice
Make sure you notice in the directions the statement "To receive incoming calls, log into your Google Voice account and enable the “Forward to Google Chat” option. You must also be signed out of the chat feature in Gmail.".