Every phone is different. Some automatically do this, others give you a choice. I checked with colleagues, and your phone should use WiFi as the default whenever it sees a network that has already been added. So you should be good.
However, they mentioned that somewhere in the WiFi or advanced settings there is a place where you can control when Wifi goes into sleep mode.
wi fi settings > menu button > advanced-> wi fi sleep policy
Some people have reported issues that if it is set for Wifi to sleep when the screen goes blank, it doesn't always reconnect to Wifi when you use the phone again. They recommend that if you have this issue you set it to never sleep. If you are mostly in Wifi areas, this actually doesn't deplete your battery much, if at all, as always searching for networks and connecting to 3G can be just as draining. In fact, he said many report improved battery life.
Hope this helps
