From what I have found, the kind of silent retreats (aside from going into the wilderness), that I think you are looking for, typically come in 2 main forms (both grounded in Buddhism). The first are those put on by places that do pretty much nothing but that for paying clients. The Insight Meditation Society has a few locations...Spirit Rock is another one on the west coast, but there are others, esp. in CA. CO has Shambhala Mountain, as mentioned, among others. These tend to be pretty pricey, although they often offer sliding scales for those with lesser means, or cheaper rates for those willing to work while they are there. These retreats feature well know teachers of mindfulness practices, and the resources are geared toward enhancing the client experiences.
The other type is monastery based visitor programs (like the Abhayagiri Monastery in CA (although they just put in a bunch of visitor facilities recently, so I'm not sure whether they are moving toward the above model???)). These are working monasteries, with some guest quarters, and often require that you work for part of each day, and suggest a donation. The daily schedules are a lot like the retreat center schedules, but are focused on the workings of the monastery and the life and practices of the monks. Guests follow along, with less resources going just to them and their experience.
Having made this distinction, I will also questions it, as there are places that straddle the two categories in many ways...but that gives you a framework to work with...(bascically, but imperfectly: monasteries vs. retreat centers).
