Hi all,
I've noticed that when I unlock the phone that the phone is in 3G mode, despite a WiFi network being available. Immediately after unlocking, WiFi is chosen, and the 3G icon goes. I lock the phone (when WiFi is active), and unlock, and 3G is again displayed for a brief second or so.

Concern is that I'm using 3G instead of WiFi when the phone is left alone. Like many I'm on a Vodafone 1G limited data usage plan with my provider; having come from an O2 Unlimited Data plan, I'm a little cautious.

I've installed a 3G data usage monitor, and its definitely going up; so my apps (facebook, email etc) are all using 3G when the phone is in standby, instead of WiFi.

Anyone noticed this? or thoughts on what's wrong?

Go to your WiFi settings -> press menu and select advanced -> go to Wi-Fi sleep policy -> select "Never".
Now your Wi-Fi will never turn off automatically when you lock your phone.

Another solution is to just turn your data transfer off.
This way you know when wi-fi turns off, you know for definite that no data is being transferred.
It will also use less battery than having wi-fi on constantly.

Thank you both; two good solutions. I've opted to disable background data transfer, and will see how that goes; although going by what I've read, some apps may still use 3G when the SGS is in standby, even though background data transfer is off.

Will see how things go. Else I'll try leaving WiFi on all the time.

Thanks again.