To clarify, Data (3G, whatever) stays on all the time, but goes into a low power state when the screen turns off. So your phone is always connected to Microsoft's servers. All push notifications, from Microsoft or otherwise, come through Microsoft servers. The amount of data that gets pushed to your phone though is minimal (email headers and text-no images, toast, etc.).
WiFi turns off 30 seconds after your screen turns off, unless you have an active connection, like streaming music or downloading a very large file. Then it will remain on till the task is complete. Note that with data turned off, WiFi will turn on every two hours, while your phone is sleep, and check for updates, push, etc.
WiFi always takes precedence over 3G. So with WiFi and your screen on, you are using that to transfer data. Some apps though, like Fox News spend on 3G for a connection and will not update automatically over WiFi. Not sure if there as others.
Prior to turning on WiFi 24/7, I was using nearly 2GB of data per month. Since turning WiFi on (with 3G still on) I consistently use 200-225MB/month. I am on an unlimited plan, but my WiFi speed are 4-5 times faster.
Actually, all well thought out and pretty energy efficient. Although, as some have noted, might mt be bad to have WiFi always on when data is disabled. Carriers probably don't like that though.
Sent from my HTC Surround using Board Express