My sump is lower than my display tank so the water flows into the sump with a power outage and not out. I have stand pipes in the overflows of my main tanks so the sumps don't overflow. You should do a test run by shutting off the return pump to see what happens. You don't want to first find out when your not at home.
set the return in the tank to a level that when you cut the power it only back siphons to a level you want in the tank. keep it toward the top and surface skim with it. its best to test!
you can also drill a hole in the return line right under the water level. ( mark the ouside of the tank with the system running so you know what the water level is)
Once the power fails it'll only drop to that small hole which will break the siphon.
Oh yeah, siphoning from the return. Doh! The above suggestions are good advice. My return is two spray bars (from two rena xp canister filters) and a jet just below the surface (from a mag 12 return pump from the sump).
I am going to use a rubber maid garbage can as a wet/dry filter for my 80g tank. I made the stand high enough to accomodate this. Lots of room for pot scrubbers between two egg crate sheets. I will have the return pump on the bottom and polyester fleece on top to catch the larger particles.