  We present two exact results for singular features in the radio frequency
intensity $I(\omega)$ for ultracold Fermi gases. First, in the absence of final
state interactions, $I(\omega)$ has a universal high frequency tail $
C\omega^{-3/2}$ for \emph{all} many-body states, where $C$ is Tan's contact.
Second, in a \emph{normal} Fermi liquid at T=0, $I(\omega)$ has a jump
discontinuity of $Z/(1 - m/m^{*})$, where $Z$ is the quasiparticle weight and
$m^*/m$ the mass renormalization. We then describe various approximations for
$I(\omega)$ in polarized normal gases. We show why an approximation that is
exact in the $n_\dn=0$ limit, fails qualitatively for $n_{\dn} > 0$: there is
no universal tail and sum rules are violated. The simple ladder approximation
is qualitatively correct for very small $n_{\dn}$, but not quantitatively.
