Strange - I've always been using my ATV3 this way (ATV3 with latest firmware is on Ethernet, MBP is on Wi-Fi (iTunes latest 10.x version always - haven't tested with the brand new 11 yet), both connected to the same AP) and have never had any problems streaming both home and protected iTunes Store movies.
I use my notebook and iTunes to play music and stream it via Home Sharing to two iHome wi-fi wireless speakers as well as my Apple TV 2. So my setup is:
Wi-fi: Notebook w/iTunes
Wi-fi: iHome wireless speaker 1
Wi-fi: iHome wireless speaker 2
Ethernet: Apple TV 2
For the first 18 months, I could do this with my ATV2 connected via ethernet cable, get the highest speed possible for Apple TV. With the prior firmware update (must have been around August or September) I lost this ability, my ATV2 can't see my notebook even though they are on the same network (it's the FiOS cable modem and router that the ATV2 is wired into for cripesakes).
When connected via ethernet, I can stream content and use AirPlay but I can't use Home Sharing. Very annoying. If I disconnect the cable, let the ATV2 connect via wi-fi, toggle Home Sharing on/off, boom it works. But I placed my FiOS router right at my Apple TV because I wanted the best speed possible. I don't want to have to use wi-fi.
BJ