3G connects to WiFi but won't use it
I've got an iPhone 3G running OS 3.1.3 (I'm not about to bring on all that 4.0 ****)
My Settings->Network->Wi-Fi pane shows that the phone has joined my LAN, shows the proper name and 5 bars. But, whether or not I have enabled 3G, the wi-fi bars never appear at the top of the phone panel. It always connects via 3G or AT&T. Wi-Fi was working recently (not sure when this problem arose, but I recall having joined the LAN within the past few weeks or so).
What could cause this? Hardware, software, some setting?