I'm thinking of booking a Delta flight from San Francisco to London, which requires changing planes in Cincinnati. The time allotted for the change-over is one hour. I understand CVG is a small airport, but is that enough time, allowing for the possibility that the plane from SFO might be late?

It's a "legal" connection, meaning you should be able to make it under normal conditions. According to flightstats.com this route has a good on time performance rating (87% on time.)
Presume you are on Delta to LGW. If there is a delay coming from SFO and you miss the connection you would go out on the next available flight - which may not be until the next day from CVG, though they might be able to get you to JFK or ATL for the late flight into Heathrow. You could also just book flight to London (LHR) from SFO through JFK or ATL with a bit more of a cushion in terms of connection time.

I think you will be fine. CVG is small and you usually don't have to wait for a gate. The only connecting problem I have is when it involves a commuter airline (Commair) and concourse c. International flights are out of concourse b. Usually 20-30 minutes is enough time to connect anywhere in CVG.

I've twice sat on the CVG-LGW plane waiting for a delayed Delta arrival from elsewhere in the US to get its passengers across to the Gatwick flight.

There's usually enough leeway in the Gatwick turnround time for it not to matter much if it leaves CVG a bit late.

One hour should be enough. Sure, if your flight is very late, it might cause a problem, but that is one of the risks of flying. I wouldn't feel more comfortable, for example, with a 2 hour connection time in ORD.

Besides, if the delays are known ahead of time, then just speak to a Delta agent and see if they can re-route you through MSP, DTW, ATL, JFK, or on another airline.

bibiras - do you "have to" take Delta ? I would not take the risk. In the event you make the connection in CVG, with just a little delay from SFO your bags may not make it.
I would take a BA UA or VS non-stop flight