Data access by iPhone 3G: WI-FI or Telco Network?

[mingpey]

Got a question on the data access mode used by the iPhone 3G.

Scenario: I am in a location with WI-FI access, and I activate my wireless network reception in my iPhone.  When I access Safari or any of the apps that require an online connection, am I downloading the data via the wireless network or through my telco's 3G mobile network?

Really interested to know cos I have to pay for data access via the telco mobile network, whereas most of the wireless networks I access are free of charge.

Is there a way to indicate a preference for either mode of data access depending on the situation?

thanks so much!