  We calculate the response of the microscopic Dirac spectrum to an imaginary
isospin chemical potential for QCD with two dynamical flavors in the chiral
limit. This extends our previous calculation from the quenched to the
unquenched theory. The resulting spectral correlation function in the
$\epsilon$-regime provides here, too, a new and efficient way to measure
$F_\pi$ on the lattice. We test the method in a hybrid Monte Carlo simulation
of the theory with two staggered quarks.
