Does google voice require a data plan to place calls? I own a Nexus S and will be travelling to US on a business trip. I will have a T-Mobile prepaid connection and intend to use Google Voice to make and receive calls. 

Since I am currently outside the US, I cannot determine whether GV android app requires a cellular data plan to make calls.
A little bit yes. (See comments below) The app will make this seamless but should still use a bit of data.

Edit: After checking Google Help I found this:


  By calling your own Google number
  
  When you call your own Google number
  and sign into the Google voicemail
  system, you’ll have the option to
  place a call. Just press 2 and enter
  the number.
  
  For calls within the U.S, enter area
  code and number. For example:
  212-555-2222. For international calls,
  enter 011, the country code, and the
  number (or "1 and the country code,"
  depending on where you're calling).
  For example: 011442012345612345 or
  12464260200