Hi, Steve--
Good question.
If I may re-phrase the question-- not to avoid answering it, but, rather, to get at the essential difference between what Saul did in conjuring up Samuel, and what Christians do when they talk/pray to the Saints (and so to see why Saul's act was sinful, and why talking/praying to Saints is not)-- I would simply ask the question, "Why was Saul's act sinful?" And, secondly, "Is their a difference between what Saul did and what Christians do when they talk/pray to the Saints (and does this difference affect the moral quality of the act?")
So, to answer the first question, "Why was Saul's act sinful?" It was sinful because it was an act that was (attempted to be) done outside of the presence or sight of God, and by, seemingly, relying on evil spirits to obtain contact with Samuel (for, Saul went to the witch of Endor to do this, and it is highly likely that evil spirits were, thus, involved in all of this). Thus, it was a very, very, very dark act and, thus, extremely and gravely sinful.
Second question: "Is their a difference between what Saul did and what Christians do when they talk/pray to the Saints (and does this difference affect the moral quality of the act?") Yes, there is a difference, and a rather large one, between what Saul did and what Christians do when talking/praying to the Saints. For, Saul attempted to talk to Samuel outside of the sight of God; however, when Christians speak/pray to the Saints, they do is in the very sight of the Heavenly Father who, by the grace that He, through Christ, has granted to both the Saint on earth, and to the Saint in heaven (though, for the Saint in heaven, grace has blossomed into heavenly glory), unites the two souls in His love. Secondly, while Saul relied on evil spirits to get into contact with Samuel, the Christian relies on the Holy Spirit, who, by grace, unites the Christian alive on earth to the Christian in heaven (and, though the Christian in heaven may be, temporarily, physically dead, he is more spiritually alive than even the Christian on earth). It is by the power of God, as taught above by Thomas Aquinas, that the communication between the Saints on earth and the Saints in heaven can take place. Thus, the communication between the Saints on earth and the Saints in heaven is done entirely within the presence, power, and love of the Holy Trinity-- and, thus, it is not only not sinful, but it is actually a beautiful act of charity that takes place between those who are awake, and those who now sleep, in Christ.
And, so, in the end, though there may be a similarity between what Saul did with Samuel, and what the Christian does in praying/talking to the Saints in heaven, that similarity being that there is a communication between those who live on earth and those who have physically died and moved on to the next life, there are significant differences which greatly affect the moral value of each of these acts (one being gravely sinful, the other being a virtuous act of charity and piety). So, on this matter, as in all things, we ought to "judge not by (mere) appearance, but judge with just judgment."
In Christ,
BrotherAlan
P.S.
For the record, here is the Catholic Church's teachings (condemnations) of divination and magic (from the
Catechism of the Catholic Church):
Divination and magic
2115 God can reveal the future to his prophets or to other saints. Still, a sound Christian attitude consists in putting oneself confidently into the hands of Providence for whatever concerns the future, and giving up all unhealthy curiosity about it. Improvidence, however, can constitute a lack of responsibility.
2116 All forms of divination are to be rejected: recourse to Satan or demons, conjuring up the dead or other practices falsely supposed to "unveil" the future.48 Consulting horoscopes, astrology, palm reading, interpretation of omens and lots, the phenomena of clairvoyance, and recourse to mediums all conceal a desire for power over time, history, and, in the last analysis, other human beings, as well as a wish to conciliate hidden powers. They contradict the honor, respect, and loving fear that we owe to God alone.
2117 All practices of magic or sorcery, by which one attempts to tame occult powers, so as to place them at one's service and have a supernatural power over others - even if this were for the sake of restoring their health - are gravely contrary to the virtue of religion. These practices are even more to be condemned when accompanied by the intention of harming someone, or when they have recourse to the intervention of demons. Wearing charms is also reprehensible. Spiritism often implies divination or magical practices; the Church for her part warns the faithful against it. Recourse to so-called traditional cures does not justify either the invocation of evil powers or the exploitation of another's credulity.