This is the conclusion put forth by LDS.About.com, which cites an April 1977 Ensign article entitled "The Do's in the Word of Wisdom" as evidence. Here's the applicable statement, republished with surrounding context:
Herbs also add variety to our meals through spices such as onions, garlic, peppers, sage, parsley, bay leaves, etc. Some herbs have medicinal properties as well. However, one should use extreme care in treating illnesses with herbs, since most have not been subjected to the careful testing required for other drugs and some contain powerful chemicals. Some people enjoy herbal teas, but even these should not replace more nutritious drinks in the diet. Always tell your physician if you are taking any herbal teas and know what the tea contains if possible. Recently an association has been identified between herbal teas and cancer of the esophagus, but further research is needed. In Utah, in the summer of 1976, three deaths were attributed to the overuse of herbs. When dealing with severe or chronic illnesses, be especially wary of herbal treatments or remedies that have not been proven effective through scientific studies. Priesthood blessings and competent medical care, not home remedies, should be obtained for treating serious disease.
Since Doctrine & Covenants 89:11 states “And again, verily I say unto you, all wholesome herbs God hath ordained for the constitution, nature, and use of man — every herb in the season thereof, and every fruit in the season thereof; all these to be used with prudence and thanksgiving”, it would be illogical to ban consumption of herbal tea. The teas considered banned by the Word of Wisdom are black, green, white, and oolong. One person nailed it down in this comment: "The tea prohibited by the word of wisdom is Camellia sinensis. Herbal teas not containing this ingredient are generally ok".
Another source which somewhat supports the conclusion that herbal teas are O.K. is the August 1979 Ensign article entitled "I Have A Question". In response to the question "Do herb drinks fall into the classification of 'hot drinks' forbidden by the Word of Wisdom or are they “herbs to be used with prudence and thanksgiving?”, Dr. Clifford J. Stratton, associate professor of anatomy, University of Nevada, School of Medical Sciences, who served at the time as a high councilor in the Reno Nevada North Stake, gave a more ambiguous answer, saying "That’s a good question but a difficult one to answer, since over a thousand different herbs have been identified to date but not all of them have been so thoroughly studied that we know all of their medicinal values. Fortunately, the most popular herbs and herb drinks have been analyzed—but the only source I’d recommend checking is a medical library. Information available from popular or commercial sources that I’ve examined is frequently unreliable". Dr. Stratton does provide an excellent tutorial on herbal therapy, and suggests one uses herbs with caution and prudence, seeking professional medical advice as needed.
Yet another source confirming that herbal teas are O.K. is an August 1983 Ensign article entitled "Observing the Word of Wisdom — Politely", where Elder Robert E. Wells of the First Quorum of the Seventy writes "In most countries there are now hot cereal-based beverages or herb 'teas' that are easy substitutes for coffee or tea".
So there you have it -- if it is not black, green, white, or oolong tea, and if the herb tea doesn't have Camellia sinensis listed as one of the ingredients, it is not a Word of Wisdom violation. But LDS.About.com reminds us that we ought to focus more on the spirit of the law rather than the letter of the law. The Lord has not given us a specific list of each and every kind of substance that is/isn't good for us, but has given us the agency to study it out for our own understanding and to choose how we will accept and obey the Word of Wisdom. When searching for specific answers, it is best to turn to the Lord through fasting and prayer to receive personal revelation.
Finally -- what about decaffinated coffee and non-alcoholic beer? Another LDS.About.com article implies these are not Word of Wisdom violations, stating "Drinking beer that doesn't contain alcohol would be the same [as decaffinated coffee]. It won't keep you from being a worthy member of the church, but it is advised to not drink harmful drinks. If beer doesn't contain alcohol then what does it contain that may be harmful and/or addictive".