That is related to the Adobe DE implementation. The fonts shipped with version of DE used on Sony reader do not have support for italic style.
You could do this for a change (applies only to Sony readers):
Code:
@font-face {font-family: "serif1";src:url(res:///opt/sony/ebook/FONT/tt0011m_.ttf)}
body{font-family: serif1,serif; font-style: italic}'