One aspect of this issue appears in the Journal of Halacha and Contemporary
Society, Number IV, Fall 1982, by Rabbi Friedman, p70-79.
In summary there are 3 issues in women wearing pants.
1. Beged Ish (men's garment).
According to many authorities (Bach, Taz ...) this is a problem only
if one wears the garment in order to appear as the opposite sex.
Thus, intention is a major item. According to others intention is not
important. However, even according to these authorities it is a
problem only if most of the clothing is that of the opposite sex.
It would seem that if it is obvious that it is the garment of a women
then almost all authorities would agree that there is no prohibition
to a woman wearing pants. However, there is a responsa of Minchat Yitzchak
that states that as long as it called pants that it is forbidden for a
woman. Rav Ovadiah Yosef clearly disagrees with this decision.
2. Tzniut (modesty)
Obviously tight fitting or short pants are forbidden by the rules of
modesty. In many ways loose pants can be more modest than dresses.
Some readers have mention that a dress demonstrates that the woman is
religious in much the way that a kippa identifies a man. There is
obviously more of an area of hashkafa than strict halakha. I recall
reading that when bras first came out that some rabbis opposed them
on similar grounds.
In summary I have checked several responsa on the issue including the
one by the Tzitz Eliezer and have seen no mention of problems with the
pants showing the split. If anyone has any sources I would appreciate it,
especially if they are from any source before this generation.