This (excellent!) answer mentions that a Wizard must specialize in Abjuration in order to qualify for Incantatrix. I can't find this rule - as far as I can tell, the only restriction is that the Wizard cannot have Abjuration as a banned school.
From Player's Guide to Faerûn, p. 61:
Special: The candidate cannot have abjuration as a prohibited school.
However, KRyan's expertise on the subject, plus 3.5's tendency to put important rules in weird places, leads me to wonder if this might be a rule I missed somehow.
So, does a Wizard who wants to be an Incantatrix have to specialize in Abjuration?