Last month during the ice storm our power went off at 6pm and didn't come back on until 6am the next morning. No heat, no anything in my tank for 12 hours. I was stranded in a hotel trying to get back from the Rockford frag meet or I would have been frantically trying just about ANYTHING! I fell asleep in my hotel room (with my new frags and RTA) realizing that there wasn't a thing I could do and I would probably loose it all.
I do not know how cold the tank got since I didn't make it home until almost 11am the next morning and the heat had been on for 4 hours at that point. I didn't loose a thing (thankfully!) but I did have some black spots and patches on the rock that are just now going away (dead algae or something?) I am guessing that the tank water was easily in the low 60's if not lower since my husband went to sleep with his mittens on.
Also, when I bought my current setup (Jan 1st of 06) we moved it all in buckets, the water and the fish from 60 miles away. I know that it was 62 degrees when I reassembled it at home and I didn't loose a fish. I really didn't know at the time how risky a venture it was. Maybe these fish and critters are tougher than we give them credit for, especially if it happens slowly (such as the temperature change.)
Just my experience, I hope it makes you feel better!