Perennialism is the name of an school of thought which has become popular among some Muslims, especially in the West, this past century. Some of the prominent figures in this school are Rene Guenon, Titus Burckhardt, Martin Lings, Charles le Gai Eaton, Seyyed Hossein Nasr and Frithjof Schuon. (There are other people involved in the larger movement which includes Hindus, Christians, and even some straight-up Fascists, but the above names are the most prominent among those who claimed to be Muslim.)
The basic idea is that there is a single body of truth known as the perennial wisdom which human beings have known from a very early point in history and which gets restated over and over again in the world's major religious traditions. So, for example, Muhammad (saaws), Jesus (as), Krishna, Zoroaster etc. were all basically teaching the same eternal principles, except in a language appropriate to the needs of their audience.
(In some sense this is similar to the Bahai faith which also affirms a belief in the Buddha, Muhammad, Jesus, Zoroaster, Krishna, and others. Although one difference is that the Bahai faith has a notion of "progressive revelation" which develops a time-based hierarchy among the religions. And since the Bahai faith is the most recent, it sits at the top of the hierarchy before those which came before)
You might think this suggests some kind of cafeteria-style spiritual free-for-all but the above names were also part of a movement known as
Traditionalism which asserted that this perennial wisdom is best expressed in the traditional, authentically-transmitted, orthodox versions of the various religions. So even if at a very deep level, the various religions agree, it still isn't appropriate to mix and match. Each path should be taken seriously, and followed on its own terms.
So the Perennialists are interesting because, on the one hand, they seem to see a great deal of value in other religions and other civilizations. But on the other hand, they also strive, in their own way, to stay faithful to a particular religious tradition and stay "orthodox".
Some members of the movement, especially in its "Traditionalist" form really emphasize the past and see the modern-age as decadent with things only getting worse. (I have the impression that this is what really motivates the scarier Fascist-Traditionalists)
Mark Sedgewick has a
website which goes into a certain amount of depth on Traditionalism and its different manifestations (Actually that first page is an interesting article on the introduction of sufism to the west, but if you go to the homepage you'll find more ino). Sedgwick also has a
paper called "Marginal Muslims in Cyberspace"
You can also go to
Religio Perennis which is another website which deals with these ideas, but more from an insider's perspective.
And as a counter-balance to give another side of the issue, Dr. Muhammad Legenhausen wrote a piece called
"Why I am not a Traditionalist" for a pretty thorough discussion of Traditionalism faults from the point of view of a Muslim who rejects it (rather thoughtfully I should add)
Also at the Living Islam site you can find a
page which doesn't deal with these terms head-on but discusses issues in the same ballpark from a critical Islamic perspective.