The idea of illusion and reality has been in a few of Shakespeare's plays (think Macbeth and A Midsummer Night's Dream). In regards to The Tempest, as vangoghfan points out, illusion is used to change reality. Those who come to the island are simply challenged in regards to their examination and understanding between the two.
One way of looking at this question is to consider the ways in which illusion is used (especially by Prospero and Ariel) to change reality -- to promote transformations, especially in the characters who find themselves on the island. In this respect, their artistry resembles that of Shakespeare, who used his own theatrical illusions to help alter the reality of the persons who visited the theater.