I confirming that I have the same problem. When the device has been using a 3G/EDGE connection, it occasionally fails to reconnect to WiFI. The only remedy I (and others, it seems) have found, is to reboot the device. After that it works normally.

However, I've noticed that I can connect to unprotected WLANs just fine when this happens. I'm using WPA2. Perhaps this has something to do with this issue?

I wonder if we should file a bug report about this?