When tomato plants grow vigorously in mild, spring weather, the top growth often exceeds the root development. When the first few days of warm, dry summer weather hit, the plant 'realises' it has a problem and needs to increase root development. The plant tries to reduce it's leaf area by rolling leaves. The leaves curl along the length of the leaf (leaflet) in an upward fashion. It is often accompanied by a thickening of the leaf giving it a leathery texture. Interestingly, leaf roll is worse on some varieties than others.
This is just a simple case of slight overwatering, nothing to worry about, if you just ease off on the watering a little. At this time of the year it is usually on lower leaves, just try to water more evenly will have a smaller root system. Saying this, under-watering can give you blossom end rot.
This problem usually applies to tomatoes. Unlike potatoes, the curling does not indicate a disease. Inward rolling of young tomato leaves is usually taken as a good sign if the leaves are dark green. The rolling of older leaves is a sign that too much foliage has been removed from the plant or a wide variation of temperatures. The plant shown here looks like it has taken up excessive nutrients, which could be in the compost. It will settle down and should grow out of this.