This is actually a common occurrence when there is a fluctuation in watering.  Cracking and splitting usually happens in the later stages of growth when the tomato is beginning to ripen.  The tomato skin will thicken and toughen up when there is a dry spell.  When a sudden influx of water occurs the tomato continues to grow inside and will cause the thickened outer skin to rupture resulting in a split or crack.  Minor cracking at the top is by and large harmless.  Keep in mind that large cracking or splits may expose the tomato to disease and insects.