As of early 2011, only St. Basil's School of Gregorian Chant in Houston, Texas (in association with the University of St. Thomas) has a regular, active program designed for learning the musical form. The school's primary goal is the preservation, enhancement and singing of Gregorian Chant in the Novus Ordo of the Sacred Liturgy. St. Basil's also focuses on history, discipline, spirituality and comprehensive learning.
St. Basil's puts on frequent classes and workshops taught by some of the greatest scholars on the subject, often featuring guest conductors. Classes focus not only on the singing, but also the history of how the chant became used, as well as its use over the last 150 years. Lessons include English versions of the chant and instructions on singing the Benedictine Compline.
Classes at St. Basil's are run seasonally and often take the form of four day workshops, the first of which is a short class that's open to the public. The second day includes history of the chant, along with its restoration and a master class. The third day consists of more contemporary information and singing. The fourth and final day focuses primarily on the preparation and singing of several different pieces.
If you don't live near Houston, there are other alternatives to learn Gregorian Chant including books, on-line courses and community sources. Websites like Amazon.com have hundreds of books and CD's on the subject. On-line options include The Chant Kit, which is available for purchase and includes 43 professionally recorded chants plus a booklet that teaches notation, pronunciation and rhythm. Your local community may also have resources available. Try asking around churches in your area to see if any of them offer classes or concerts. You can also try Societies for Creative Anachronism.