Technical: November 2008 Archives
There is none. Stop looking!
Neither W3C Schema's or Relax NG are capable of describing the the Atom specification so if you want a compliant validator you'll need to write your own.
For a thread on the issue see:
No W3C Schema for Atom
I also ran into an interesting thread where James Clark puts forward Relax NG as a serious alternative to W3C Schema.
Relax NG vs W3C Schema
Having dealt with both I can say I prefer Relax NG. Its simpler.