If you want to ensure you're just using wifi, turn off 3G and Edge using the method below:
1. Hold down the on/off button for a few seconds until the 'Phone Options' pops up.
2. Select 'Mobile network' and ensure it says 'Connect to Internet is OFF'.
This will turn 3G/Edge off, leaving just WI-FI when available and your network operator.
As said above, the Hero will connect using WI-FI first, then if thats not available it will use 3G... unless you do the above.