Women ARE serving in the front lines, both in Iraq and Afghanistan, simply because they are wars with no front lines.
That being said, i'm sure there are women who would perform excellently under fire and in combat. However, you also have to consider things like physical size; most women cannot run with the 50+ pounds of gear carried into combat. Likewise, women face certain medical needs that sometimes cannot be met in a combat zone.
Of course, there are other opportunities in the military. I've worked next to women who, mentally, outperform some of the men I work with.