Nicads drop off pretty fast below 1.2 volts per cell. I have taken some short flights at 4.6v takeoff. Depends alot on how much load your radio setup has. And the capacity of your batteries. Lately i have been using 1800mah nimh battery packs so i haven't really had much problem with it. Nimh discharge curves are pretty flat and yes 1.1 volts is the absolute minimum landing voltage. Here is a link to a discharge curve for nimh.
http://www.digibattery.co.uk/nimh_tech.html
And a battery guru's take on it.
http://www.rcbatteryclinic.com/ (look in the loaded digital voltmeter section).[&:]

edit: individual radio systems may not work at that level-it's up to you to test it on the ground.