Can someone advise me whether the Blackberry 8520 Curve actually has a case that helps the battery life.

I have been told that this model is the only Blackberry that doesn't work in this way.

Is this right?

Ive not heard that before.
You need an OEM case that contains a magnet.
This magnet puts the device to sleep.

I put my 8520 in my old 8700 case to test it. It works the same way as the 8700 did. The magnet turns off the backlight, saving the battery. I got one of those skins from the T-Mobile store for the 8520, and with it on it fits the 8700 case perfectly, saving me money.