When Chazal discuss their views on matters relating to the physical world, which have now been rendered obsolete, is this still considered to be Torah? May one study it in the bathroom? Must one say birchas haTorah before studying it? Or is it bittul Torah to spend time on it?
There are different categories to discuss here. One is scientific information that does not relate to Scripture, halachah or theology in any way; for example, the medical remedies in the Gemara. If the medical texts of Rambam are not considered Torah, why would the medical texts of Chazal be considered Torah?
Another category would be scientific information that Chazal attempt to derive from Scripture - for example, the exegeses from Scripture via which Chazal deduce that the sun travels behind the sky or along the horizon at night, and the exegeses via which they deduce the nature of the "firmament."
Of course, the answer to this question depends on how you define "Torah." Is Torah that which was given at Sinai? Or is "Torah" man's attempt to understand and develop that which was given at Sinai? The first definition seems a little narrow; but the second definition is somewhat ambiguous.
Rav Moshe Shapiro commented to a number of people that if anything in the Gemara was nothing more than obsolete scientific beliefs, it would be bittul Torah to learn it. Since we know that people such as the Vilna Gaon were virtually never mevattel Torah, it must be that Chazal were never merely discussing science. Accordingly, everything in the Gemara is discussing metaphysical matters, and is Torah.
But this, of course, stands in contrast to the opinion of most (and perhaps all) Geonim and Rishonim.
Rabbi Moshe Meiselman claims that when Chazal related mistaken beliefs about cosmology or spontaneous generation, they were merely offering their own opinion on scientific matters, which were therefore fallible, rather than expounding the Torah. I'm not sure if he therefore considers that these things are not Torah. But in any case, his position is inherently flawed, because Chazal did indeed invoke derashos for their views on cosmology and spontaneous generation.
So what's the bottom line? Personally, I have no idea.