When you are using wifi, you are not counting against your plans data limits, but do be aware, that you may not be using wifi as much as you think you are. If you are in a wifi area, but you allow your phone to go into standby mode, it will rutn off the wifi radio and revert back to using the cellular data network (which does count against your plans data limits).
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
Note that not all features are covered through WiFi. Cellular based features including your voicemail, sms/mms, and the phantom data service will ping for 3G data now and then. You can go to settings/general/network/data off but you will not receive any of these cellular based features until it is reactivated.