What possible issues have to be considered when we look at this new trend.
1] Some sources would put this under the prohibition of a woman wearing men's clothing
2] Rabbi Rosner in one of his lectures said that Tefillin requires a clean body, if you don't therefore have the outright obligation to wear them (men do, women don't) he holds it is better for women not to wear Tefillin, because the potential negatives outweigh the benefits.
3] One at least has to wonder if the motivation for Tefillin by certain womens groups stem more for a desire to make a Woman's Lib statement.
4] The issue of Minhag also has to be dealt with