I was worried abt the same thing since the network charges exorbitant rates for internet usage... When WIFI is not available and I accidentally click on the browser, the phone automatically reverts to the 3G network (and if you clicked on the notification bar, you'd see a timer showing how long you'd been connected) even if I had Data Connection off (Settings>Communication>Data Connection).

In the end I decided to change which networks are automatically used. (Settings > Connections tab > Connections > Advanced tab > Select Networks > Programs that automatically connect to the internet should connect using) By default there are unedited options in the dropdown list like "My ISP name", "My Work Network", together with your 3G network's name. These default options unedited can't actually connect you to the Internet, so I use them as dummy networks to prevent any accidental connection to the 3G network. When I do want to use the 3G network, I simply change above option and select the appropriate network from the dropdown menu. It's a novice solution but it works for me and I sleep a lot better because of it. ;p

(Oh and when I turn on WIFI and connect to a wi-fi network, that connection is automatically used, so this solution doesn't affect WI-FI negatively)

Try this:
From you All Programs page, open Windows Media Player.
Tap on "Menu" and then "Update Library".

Try to find and install Old Sap Don`t sleep wifi app, that should be resolve wifi dissconnection