You can read all about R. Slifkin's heresy here,or you can go straight to this wonderful article, in Dei'ah veDibur. An excerpt:
Gedolei Yisroel [the greatest living rabbis]* have issued an open letter against books by Rabbi Nosson Slifkin (also known as "the Zoo Rabbi") such as The Science of Torah and Mysterious Creatures, which are filled with heretical ideas on the fundamentals of emunoh [faith] formed by the former yeshiva student himself.
When these books reached the hands of English-speaking talmidei chachomim [religious scholars] they were shocked and dismayed at the contents. Several months ago HaRav Yitzchok Sheiner, rosh yeshiva [headmaster] of Yeshivas Kamenitz and a member of Moetzes Gedolei HaTorah, wrote a letter to gedolei hador [the greatest rabbis of the present generation] in which he said, "As one who knows the English language I hereby testify before you that a talmid chochom [scholar] concerned for the honor of Torah and emunoh came to me and showed me books written by an individual by the name of Nosson Slifkin, which are hair-raising to read. Perhaps I transgressed the obligation to tear kria ["tearing,"** a mourning ritual] when confronted with things that appear to be complete heresy, against all that is accepted and known according to our faith ever since the Torah was given on Mt. Sinai. He believes that the world is millions of years old—all nonsense!—and many other things that should not be heard and certainly not believed. In short these books cannot be brought into the home of one who believes in Hashem and His Torah."
To this letter HaRav Elya Ber Wachtfogel, rosh yeshiva of Yeshivas South Fallsberg, adds, "And he also writes that Chazal Hakedoshim [the authors of the Talmud] can err chas vesholom [God forbid] in worldly matters chas vesholom and therefore [they can err]*** in halochoh [Jewish law] as well chas vesholom, as he wrongly proves from maseches Horayos [a tractate of the Talmud] —all nonsense! And the whole book is filled with similar instances of total heresy. And even regarding the parts that are not total heresy, he who reads them and accepts his writings will eventually become a total heretic. And furthermore, even what is not heretical is expressed in a way only a heretic would speak." [I think that means that R. Slifkin doesn't write in Yeshivish.****]
My favorite line:
Rabbonim [rabbis] who gave their haskomoh [endorsement] to the book retracted their approbation in a letter explaining that they relied primarily on the fact the author was the product of yeshivas.
Now we know how much those haskomos are worth (not that there were really any illusions).
*The bracketed additions are mine, with one exception.
**A typical Yeshivish redundancy.
***This is the exception.
****This, of course, is good news for anyone who needed the information contained in the brackets.
UPDATE: Gil Student has a wonderful post with some inside information. (The comments are also worth reading.) DovBear has posted an "action alert" calling for the defense of R. Slifkin. DH has a few words on a related matter.
UPDATE 2: Interesting sociological analysis by the Hedyot (reactions?) and a shpoof.
Cross-Currents' Yitzchok Alderstein comments, and Paul Shaviv responds.
Looks like this is it for Gerald Schroeder. Welcome to the club.