3G connects to WiFi but won't use it

[lundo]

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?

 

[lungho]

Is this your home network or a work/school/public network you are trying to connect to? Is your phone jailbroken.......sometimes jailbreak breaks things like wifi.

 

[lundo]

This is a home network. The iPhone always connected and apps used the LAN via WiFi until a while ago. Standard phone, no jailbreak!