Among the new entries in the Associated Press Stylebook for 2008 is one on "myriad." The AP says that "myriad" is an adjective and is not followed by "of." The dictionary that AP uses, though, gives the noun use of "myriad" first.
"Myriad" means an indefinitely large number; it is a synonym of "innumerable." Bryan A. Garner writes in A Dictionary of Modern American Usage that "myriad is more concise as an adjective than as a noun." Fowler's Modern English Usage points out that the word comes from Greek for "ten thousand." Almost no one adheres to that old meaning for "myriad."
Here is a post on The Mavens' Word of the Day about myriad as a noun. The American Book of English Usage also points out the long history of myriad as a noun. Merriam-Webster online also recognizes myriad as a noun.
As for me, I will use "myriad" as an adjective. I'd rather not fight about it.