Does Skype/Google Voice use 3G or wifi on my iPhone?
Skype and Google Voice act very differently in the way that calls are received on your phone.  Skype is VOIP (Voice Over IP) software, and all calls made using Skype will use your data plan when you're not in a WiFi area.  However, if you have Skype Call Forwarding configured to forward to your iPhone you will be charged per minute just like a regular cell phone call.

Google Voice, on the other hand, will forward calls placed to your Google Voice phone number to your iPhone if you have that option selected.  Inversely, calls made via Google Voice will be placed through your Google Voice phone number.  The end result being, you're making a phone call using your phone's data plan.

If you're concerned about using your cell phone minutes, and are on AT&amp;T you can optionally enable the A-List service for a small monthly fee.  If you add your Google Voice number to your A-List, all calls made to and from that number will not be deducted from your regular cell phone plan.