I have a Lumia 610, running on WP 7.8
For some reason, whenever I turn off the screen, my WiFi keeps getting disconnected. This isn't in the settings. Any way to fix this?
This is intentional, to save battery, and cannot be configured to behave differently on Windows Phone 7.x. As soon as you turn the screen back on, it should attempt to reconnect to known WiFi connections, and carry on using that.
It shouldn't affect any apps, as these also get suspended automatically, when the screen turns off, unless they request permission to run "under the lock screen" - which shows up in the Store when you install the app.
You can use an app like: Keep Alive Wifi: http://windowsphonehacker.com/articles/experimental_keep_alive_for_wifi-08-10-12
I am not sure if you needed a rooted/unlocked phone for that, but I use(d) the app and it worked pretty good :)